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Hello all,
A single Windows 11 client running Nextcloud Desktop 4.0.4 cannot upload any files to a self-hosted Nextcloud 32 server over OpenVPN, while all other clients work normally. The client discovers changes but every upload fails instantly with HTTP status 0 / QNetworkReply::UnknownNetworkError, and no PUT request ever reaches the server. The issue persists after reinstalling the client, disabling firewall/Defender, and even after switching OpenVPN from unstable TCP/tun to stable UDP/tun (MTU 1400, mssfix 1360). This strongly suggests a transport-layer or Windows networking/MTU interaction that aborts HTTPS uploads before they reach Nextcloud, and I’m looking for anyone who has seen a similar Windows 11 + OpenVPN + Nextcloud client behavior.
Thanks a lot for your help - I have been debugging this already for days but without much progress.
The Basics
- Nextcloud Server version (e.g., 29.x.x):
32.0.5.0
- Operating system and version (e.g., Ubuntu 24.04):
Debian 13
- Web server and version (e.g, Apache 2.4.25):
Apache 2.4.66
- Reverse proxy and version _(e.g. nginx 1.27.2)
N/A
- PHP version (e.g, 8.3):
8.2.29
- Is this the first time you’ve seen this error? (Yes / No):
Yes
- When did this problem seem to first start?
Client never synced (uploaded) since the initial installation of the Nextcloud client
- Installation method (e.g. AlO, NCP, Bare Metal/Archive, etc.)
Bare Metal
- Are you using CloudfIare, mod_security, or similar? (Yes / No)
No
Summary of the issue you are facing:
One specific Windows 11 client using Nextcloud Windows client 4.0.4 cannot upload/sync files to a self-hosted Nextcloud server.
-
Discovery works
-
Files are detected as
CSYNC_INSTRUCTION_NEW (Up) -
Sync shows “Sync running” indefinitely
-
Upload attempts fail with:
HTTP status 0
QNetworkReply::UnknownNetworkError
"Der Schreibvorgang konnte nicht ausgeführt werden"
No PUT request ever reaches the server:
-
no
/remote.php/dav/uploads/... -
no write entries in Nextcloud audit log
-
only GET requests appear
All other clients (including other Windows clients) work normally.
Nextcloud is only reachable via OpenVPN.
OpenVPN server details:
-
OpenVPN 2.6.14 on Debian 13
-
tun-mtu 1400 -
mssfix 1360
Originally OpenVPN was TCP/tun. On the problematic client only, the VPN connection repeatedly dropped and reconnected. After switching to a UDP/tun instance, the VPN connection is now stable, but the Nextcloud upload problem persists.
Steps to replicate it (hint: details matter!):
-
- Connect via OpenVPN (UDP/tun instance)
-
Start Nextcloud Windows client 4.0.4 on the affected Windows 11 machine
-
Add or modify a file in the synced folder
-
Sync starts and discovers the file
-
Upload attempt is initiated
-
Upload fails with HTTP status 0 and UnknownNetworkError
-
Sync remains in “Sync running” state indefinitely
Log entries
Nextcloud
Please provide the log entries from your Nextcloud log that are generated during the time of problem (via the Copy raw option from Administration settings->Logging screen or from your nextcloud.log located in your data directory). Feel free to use a pastebin/gist service if necessary.
Auszug aus audit.log zum User:
{"reqId":"FMb1xtmLmbKSABkCjuJ8","level":1,"time":"2026-01-31T07:46:43+00:00","remoteAddr":"10.10.5.3","user":"--","app":"admin_audit","method":"PROPFIND","url":"/remote.php/dav/files/comp-myusername/","scriptName":"/remote.php","message":"Login attempt: \"comp-myusername\"","userAgent":"Mozilla/5.0 (Windows) mirall/4.0.4 (build 20251215) (Nextcloud, windows-10.0.22621 ClientArchitecture: x86_64 OsArchitecture: x86_64)","version":"32.0.5.0","clientReqId":"3c7e2302-bff4-4ddf-9706-b2c5256436e5","data":{"app":"admin_audit"}}
{"reqId":"FMb1xtmLmbKSABkCjuJ8","level":1,"time":"2026-01-31T07:46:44+00:00","remoteAddr":"10.10.5.3","user":"--","app":"admin_audit","method":"PROPFIND","url":"/remote.php/dav/files/comp-myusername/","scriptName":"/remote.php","message":"Login attempt: \"comp-myusername\"","userAgent":"Mozilla/5.0 (Windows) mirall/4.0.4 (build 20251215) (Nextcloud, windows-10.0.22621 ClientArchitecture: x86_64 OsArchitecture: x86_64)","version":"32.0.5.0","clientReqId":"3c7e2302-bff4-4ddf-9706-b2c5256436e5","data":{"app":"admin_audit"}}
{"reqId":"FMb1xtmLmbKSABkCjuJ8","level":1,"time":"2026-01-31T07:46:44+00:00","remoteAddr":"10.10.5.3","user":"comp-myusername","app":"admin_audit","method":"PROPFIND","url":"/remote.php/dav/files/comp-myusername/","scriptName":"/remote.php","message":"Login successful: \"comp-myusername\"","userAgent":"Mozilla/5.0 (Windows) mirall/4.0.4 (build 20251215) (Nextcloud, windows-10.0.22621 ClientArchitecture: x86_64 OsArchitecture: x86_64)","version":"32.0.5.0","clientReqId":"3c7e2302-bff4-4ddf-9706-b2c5256436e5","data":{"app":"admin_audit"}}
Web Browser
If the problem is related to the Web interface, open your browser inspector Console and Network tabs while refreshing (reloading) and reproducing the problem. Provide any relevant output/errors here that appear.
N/A
Web server / Reverse Proxy
The output of your Apache/nginx/system log in /var/log/____:
N/A
Configuration
Nextcloud
The output of occ config:list system:
{
"system": {
"instanceid": "***REMOVED SENSITIVE VALUE***",
"passwordsalt": "***REMOVED SENSITIVE VALUE***",
"secret": "***REMOVED SENSITIVE VALUE***",
"trusted_domains": [
"cloud.company.de"
],
"datadirectory": "***REMOVED SENSITIVE VALUE***",
"dbtype": "mysql",
"version": "32.0.5.0",
"overwrite.cli.url": "https:\/\/cloud.company.de",
"dbname": "***REMOVED SENSITIVE VALUE***",
"dbhost": "***REMOVED SENSITIVE VALUE***",
"dbport": "",
"dbtableprefix": "oc_",
"mysql.utf8mb4": true,
"dbuser": "***REMOVED SENSITIVE VALUE***",
"dbpassword": "***REMOVED SENSITIVE VALUE***",
"installed": true,
"maintenance": false,
"maintenance_window_start": 1,
"default_phone_region": "DE",
"mail_smtpmode": "smtp",
"mail_sendmailmode": "smtp",
"mail_from_address": "***REMOVED SENSITIVE VALUE***",
"mail_smtphost": "***REMOVED SENSITIVE VALUE***",
"mail_smtpport": "587",
"mail_domain": "***REMOVED SENSITIVE VALUE***",
"mail_smtpauth": 1,
"mail_smtpname": "***REMOVED SENSITIVE VALUE***",
"mail_smtppassword": "***REMOVED SENSITIVE VALUE***",
"forbidden_filename_basenames": [
"con",
"prn",
"aux",
"nul",
"com0",
"com1",
"com2",
"com3",
"com4",
"com5",
"com6",
"com7",
"com8",
"com9",
"com\u00b9",
"com\u00b2",
"com\u00b3",
"lpt0",
"lpt1",
"lpt2",
"lpt3",
"lpt4",
"lpt5",
"lpt6",
"lpt7",
"lpt8",
"lpt9",
"lpt\u00b9",
"lpt\u00b2",
"lpt\u00b3"
],
"forbidden_filename_characters": [
"<",
">",
":",
"\"",
"|",
"?",
"*",
"\\",
"\/"
],
"forbidden_filename_extensions": [
" ",
".",
".filepart",
".part"
],
"loglevel": 0,
"filelocking.enabled": "true",
"memcache.distributed": "\\OC\\Memcache\\Redis",
"memcache.locking": "\\OC\\Memcache\\Redis",
"redis": {
"host": "***REMOVED SENSITIVE VALUE***",
"port": "6379"
},
"memcache.local": "\\OC\\Memcache\\APCu",
"log.condition": {
"apps": [
"admin_audit"
]
}
},
"apps": {
"activity": {
"enabled": "yes",
"installed_version": "5.0.0-dev.0",
"types": "filesystem"
},
"admin_audit": {
"enabled": "yes",
"installed_version": "1.22.0",
"logfile": "\/home\/www\/account123\/company.de\/cloud\/files\/audit.log",
"types": "logging"
},
"app_api": {
"enabled": "yes",
"installed_version": "32.0.0",
"types": ""
},
"backgroundjob": {
"lastjob": "4"
},
"bruteforcesettings": {
"enabled": "yes",
"installed_version": "5.0.0-dev.0",
"types": ""
},
"circles": {
"enabled": "yes",
"installed_version": "32.0.0",
"loopback_tmp_scheme": "https",
"maintenance_run": "0",
"maintenance_update": "{\"3\":1772379601,\"2\":1772382602,\"1\":1772382602}",
"migration_22": "1",
"migration_run": "0",
"types": "filesystem,dav"
},
"cloud_federation_api": {
"enabled": "yes",
"installed_version": "1.16.0",
"types": "filesystem"
},
"comments": {
"enabled": "yes",
"installed_version": "1.22.0",
"types": "logging"
},
"contactsinteraction": {
"enabled": "yes",
"installed_version": "1.13.1",
"types": "dav"
},
"core": {
"backgroundjobs_mode": "cron",
"emailTestSuccessful": "1",
"installedat": "1727797084.3711",
"lastcron": 1772382602,
"lastupdateResult": "{\"version\":\"32.0.6.1\",\"versionstring\":\"Nextcloud 32.0.6\",\"url\":\"https:\\\/\\\/download.nextcloud.com\\\/server\\\/releases\\\/nextcloud-32.0.6.zip\",\"web\":\"https:\\\/\\\/docs.nextcloud.com\\\/server\\\/32\\\/admin_manual\\\/maintenance\\\/upgrade.html\",\"changes\":\"https:\\\/\\\/updates.nextcloud.com\\\/changelog_server\\\/?version=32.0.6\",\"autoupdater\":\"1\",\"eol\":\"0\"}",
"lastupdatedat": 1772343318,
"metadataGenerationDone": true,
"moveavatarsdone": "yes",
"newUser.sendEmail": "yes",
"previewsCleanedUp": "1",
"public_files": "files_sharing\/public.php",
"vendor": "nextcloud",
"files_metadata": {
"photos-original_date_time": {
"value": null,
"type": "int",
"etag": "",
"indexed": true,
"editPermission": 0
},
"photos-exif": {
"value": null,
"type": "array",
"etag": "",
"indexed": false,
"editPermission": 0
},
"photos-ifd0": {
"value": null,
"type": "array",
"etag": "",
"indexed": false,
"editPermission": 0
},
"photos-size": {
"value": null,
"type": "array",
"etag": "",
"indexed": false,
"editPermission": 0
},
"blurhash": {
"value": null,
"type": "string",
"etag": "e45275e8585d94f916a680141aa38948",
"indexed": false,
"editPermission": 0
},
"photos-gps": {
"value": null,
"type": "array",
"etag": "",
"indexed": false,
"editPermission": 0
},
"photos-place": {
"value": null,
"type": "string",
"etag": "",
"indexed": true,
"editPermission": 0
}
},
"oc.integritycheck.checker": []
},
"dashboard": {
"enabled": "yes",
"installed_version": "7.12.0",
"types": ""
},
"dav": {
"buildCalendarReminderIndex": "yes",
"buildCalendarSearchIndex": "yes",
"builtSocialSearchIndex": "yes",
"chunks_migrated": "1",
"enabled": "yes",
"hasCustomDefaultContact": false,
"installed_version": "1.34.2",
"regeneratedBirthdayCalendarsForYearFix": "yes",
"types": "filesystem"
},
"federatedfilesharing": {
"enabled": "yes",
"installed_version": "1.22.0",
"types": ""
},
"federation": {
"enabled": "yes",
"installed_version": "1.22.0",
"types": "authentication"
},
"files": {
"enabled": "yes",
"installed_version": "2.4.0",
"mimetype_version": "32.0.0.13",
"types": "filesystem"
},
"files_downloadlimit": {
"enabled": "yes",
"installed_version": "5.0.0-dev.0",
"types": ""
},
"files_pdfviewer": {
"enabled": "yes",
"installed_version": "5.0.0-dev.0",
"types": ""
},
"files_reminders": {
"enabled": "yes",
"installed_version": "1.5.0",
"types": ""
},
"files_sharing": {
"enabled": "yes",
"installed_version": "1.24.1",
"types": "filesystem"
},
"files_trashbin": {
"enabled": "yes",
"installed_version": "1.22.0",
"types": "filesystem,dav"
},
"files_versions": {
"enabled": "yes",
"installed_version": "1.25.0",
"types": "filesystem,dav"
},
"firstrunwizard": {
"enabled": "yes",
"installed_version": "5.0.0-dev.0",
"types": ""
},
"logreader": {
"enabled": "yes",
"installed_version": "5.0.0-dev.0",
"types": "logging"
},
"lookup_server_connector": {
"enabled": "yes",
"installed_version": "1.20.0",
"types": "authentication"
},
"nextcloud_announcements": {
"enabled": "yes",
"installed_version": "4.0.0-dev.0",
"pub_date": "Thu, 24 Oct 2019 00:00:00 +0200",
"types": "logging"
},
"notifications": {
"enabled": "yes",
"installed_version": "5.0.0-dev.0",
"types": "logging"
},
"oauth2": {
"enabled": "yes",
"installed_version": "1.20.0",
"types": "authentication"
},
"password_policy": {
"enabled": "yes",
"installed_version": "4.0.0-dev.0",
"types": "authentication"
},
"photos": {
"enabled": "yes",
"installed_version": "5.0.0-dev.1",
"lastPlaceMappedUser": "comp-otherusername",
"lastPlaceMappingDone": "true",
"types": "dav,authentication"
},
"privacy": {
"enabled": "yes",
"installed_version": "4.0.0-dev.0",
"types": ""
},
"profile": {
"enabled": "yes",
"installed_version": "1.1.0",
"types": ""
},
"provisioning_api": {
"enabled": "yes",
"installed_version": "1.22.0",
"types": "prevent_group_restriction"
},
"recommendations": {
"enabled": "yes",
"installed_version": "5.0.0-dev.0",
"types": ""
},
"related_resources": {
"enabled": "yes",
"installed_version": "3.0.0-dev.0",
"types": ""
},
"richdocuments": {
"disable_certificate_verification": "yes",
"doc_format": "ooxml",
"enabled": "yes",
"installed_version": "9.0.2",
"public_wopi_url": "https:\/\/cloud.company.de",
"types": "filesystem,prevent_group_restriction",
"wopi_allowlist": "127.0.0.1,192.12.18.90,10.10.13.200",
"wopi_url": "https:\/\/cloud.company.de\/apps\/richdocumentscode\/proxy.php?req="
},
"richdocumentscode": {
"enabled": "yes",
"installed_version": "25.4.702",
"types": ""
},
"serverinfo": {
"cached_count_appdata_files": 5043,
"cached_count_filecache": 9414,
"cached_count_storages": 15,
"enabled": "yes",
"installed_version": "4.0.0-dev.0",
"size_appdata_storage": -1,
"types": ""
},
"settings": {
"appstore-fetcher-lastFailure": "1772343438",
"enabled": "yes",
"installed_version": "1.15.1",
"types": ""
},
"sharebymail": {
"enabled": "yes",
"installed_version": "1.22.0",
"types": "filesystem"
},
"support": {
"SwitchUpdaterServerHasRun": "yes",
"enabled": "yes",
"installed_version": "4.0.0-dev.0",
"types": "session"
},
"survey_client": {
"enabled": "yes",
"installed_version": "4.0.0-dev.0",
"types": ""
},
"systemtags": {
"enabled": "yes",
"installed_version": "1.22.0",
"types": "logging"
},
"text": {
"enabled": "yes",
"installed_version": "6.0.1",
"types": "dav"
},
"theming": {
"enabled": "yes",
"installed_version": "2.7.0",
"types": "logging"
},
"twofactor_backupcodes": {
"enabled": "yes",
"installed_version": "1.21.0",
"types": ""
},
"updatenotification": {
"core": "32.0.6.1",
"enabled": "yes",
"installed_version": "1.22.0",
"richdocuments": "9.0.3",
"richdocumentscode": "25.4.902",
"types": "",
"update_check_errors": 0
},
"user_status": {
"enabled": "yes",
"installed_version": "1.12.0",
"types": ""
},
"viewer": {
"enabled": "yes",
"installed_version": "5.0.0-dev.0",
"types": ""
},
"weather_status": {
"enabled": "yes",
"installed_version": "1.12.0",
"types": ""
},
"webhook_listeners": {
"enabled": "yes",
"installed_version": "1.3.0",
"types": "filesystem"
},
"workflowengine": {
"enabled": "yes",
"installed_version": "2.14.0",
"types": "filesystem"
}
}
}
Apps
Enabled:
- activity: 5.0.0-dev.0
- admin_audit: 1.22.0
- app_api: 32.0.0
- bruteforcesettings: 5.0.0-dev.0
- circles: 32.0.0
- cloud_federation_api: 1.16.0
- comments: 1.22.0
- contactsinteraction: 1.13.1
- dashboard: 7.12.0
- dav: 1.34.2
- federatedfilesharing: 1.22.0
- federation: 1.22.0
- files: 2.4.0
- files_downloadlimit: 5.0.0-dev.0
- files_pdfviewer: 5.0.0-dev.0
- files_reminders: 1.5.0
- files_sharing: 1.24.1
- files_trashbin: 1.22.0
- files_versions: 1.25.0
- firstrunwizard: 5.0.0-dev.0
- logreader: 5.0.0-dev.0
- lookup_server_connector: 1.20.0
- nextcloud_announcements: 4.0.0-dev.0
- notifications: 5.0.0-dev.0
- oauth2: 1.20.0
- password_policy: 4.0.0-dev.0
- photos: 5.0.0-dev.1
- privacy: 4.0.0-dev.0
- profile: 1.1.0
- provisioning_api: 1.22.0
- recommendations: 5.0.0-dev.0
- related_resources: 3.0.0-dev.0
- richdocuments: 9.0.2
- richdocumentscode: 25.4.702
- serverinfo: 4.0.0-dev.0
- settings: 1.15.1
- sharebymail: 1.22.0
- support: 4.0.0-dev.0
- survey_client: 4.0.0-dev.0
- systemtags: 1.22.0
- text: 6.0.1
- theming: 2.7.0
- twofactor_backupcodes: 1.21.0
- updatenotification: 1.22.0
- user_status: 1.12.0
- viewer: 5.0.0-dev.0
- weather_status: 1.12.0
- webhook_listeners: 1.3.0
- workflowengine: 2.14.0
Disabled: - encryption: 2.20.0
- files_external: 1.24.1
- suspicious_login: 10.0.0-dev.0
- twofactor_nextcloud_notification: 6.0.0-dev.0
- twofactor_totp: 14.0.0
- user_ldap: 1.23.0
Additional technical observations
-
File size tested: ~65 MB Excel file (but smaller files fail as well)
-
Path lengths shortened and simplified — no change
-
Nextcloud client reinstalled and config/cache removed (
%APPDATA%and%LOCALAPPDATA%) -
Sync DB rebuilt
-
Forced chunking via
nextcloud.cfg -
Windows Firewall disabled for test
-
Controlled Folder Access disabled
-
Windows Defender real-time protection temporarily disabled
-
No third-party antivirus or endpoint protection installed
-
Event Viewer (Defender, Security, Schannel) shows no obvious blocking events
-
Other clients over the same VPN sync normally
Current hypothesis
The upload stream seems to be aborted on the Windows 11 client or transport layer before reaching the server.
Questions
-
Has anyone seen HTTP status 0 / UnknownNetworkError with the Windows client in combination with OpenVPN?
-
Could this be an MTU / MSS / fragmentation issue affecting TLS uploads?
-
Are there known issues with Nextcloud Windows client 4.x + OpenVPN on Windows 11?
-
Are there additional debug options in the Windows client for lower-level network/TLS diagnostics?