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Sorry to hear you’re facing problems 
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In order to help you as quickly as possible, before clicking Create Topic please provide as much of the below as you can. Feel free to use a pastebin service for logs, otherwise either indent short log examples with four spaces:
example
Or for longer, use three backticks above and below the code snippet:
longer
example
here
Some or all of the below information will be requested if it isn’t supplied; for fastest response please provide as much as you can 
Nextcloud version (eg, 20.0.5): 21.03
Operating system and version (eg, Ubuntu 20.04): Ubuntu 21.04 (GNU/Linux 5.11.0-25-generic x86_64)
Apache or nginx version (eg, Apache 2.4.25): Server version: Apache/2.4.46 (Ubuntu) Server built: 2021-06-17T17:09:41
PHP version (eg, 7.4): PHP 7.4.16 (cli) (built: Jul 5 2021 13:04:38) ( NTS )
The issue you are facing:
While the file sync worked fine all the time since installation of Nextcloud last year from all machines (Windows PC, Ubuntu Laptop, Android phone, Android tablet), a few days ago it all suddenly stopped working on all machines, without knowingly having changed anything. On the Windows and Ubuntu PC’s I run the latest version of the Desktop app (always updating as soon as a new release is announced by the Desktop app itself). On the Android hardware the apps also automatically update.
The Desktop apps for Linux and Windows show the same behaviour: in Settings they show an error with the text “Server replied with an error while reading directory ‘xxxxxx’ : Verbindung beendet” (Verbindung beendet = connection closed)
or
"[Path_and_Filename]: Server replied “500 Internal Server Error” to “PUT https://[Nextcloud_web_address]/remote.php/dav/files/[Username]/[Folder_and_Filename]”
The Android app throws an error in the notifications for each file it was unable to upload:
“Hochladen fehlgeschlagen Fehlerhaft Serverkonfiguration”, which translates to
“Upload failed Incorrect server configuration”
On Android, I can see that the files get uploaded (% is increasing from 0 to 100%), but obviously get dissaproved by the server.
At the same time, the syncing of calendar, tasks, and notes work without any issues.
Is this the first time you’ve seen this error? (Y/N):Yes, since a few days
Steps to replicate it:
- Force Sync on Desktop App or wait for next refresh on Android app
The output of your Nextcloud log in Admin > Logging:
The log is not showing anything
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
<?php
$CONFIG = array (
'instanceid' => 'xxx',
'passwordsalt' => 'xxx',
'secret' => 'xxx',
'trusted_domains' =>
array (
0 => 'xxx.local',
1 => 'xxx.org',
),
'datadirectory' => '/mnt/HD1/',
'dbtype' => 'mysql',
'version' => '21.0.3.1',
'overwrite.cli.url' => 'http://xxx.local',
'dbname' => 'nextcloud',
'dbhost' => 'localhost',
'dbport' => '',
'dbtableprefix' => 'oc_',
'mysql.utf8mb4' => true,
'dbuser' => 'xxx',
'dbpassword' => 'xxx',
'installed' => true,
'app_install_overwrite' =>
array (
0 => 'cookbook',
1 => 'files_retention',
),
'log_type' => 'file',
'logtimezone' => 'Europe/Berlin',
'logfile' => '/var/log/nextcloud.log',
'loglevel' => 2,
'syslog_tag' => 'Nextcloud',
'maintenance' => false,
'filesystem_check_changes' => 1,
'memcache.local' => '\\OC\\Memcache\\APCu',
'overwriteprotocol' => 'https',
'theme' => '',
'updater.release.channel' => 'stable',
'mail_domain' => 'xxx',
'mail_from_address' => 'xxx',
'mail_smtpmode' => 'smtp',
'mail_sendmailmode' => 'smtp',
'mail_smtpauthtype' => 'LOGIN',
'mail_smtpauth' => 1,
'mail_smtphost' => 'xxx',
'mail_smtpport' => '587',
'mail_smtpsecure' => 'tls',
'mail_smtpname' => 'xxx',
'mail_smtppassword' => 'xxx',
'default_phone_region' => 'xx',
);
The output of your Apache/nginx/system log in /var/log/apache2/access.log:
I do not know what to extract from that large file.
PASTE HERE