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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
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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, 12.0.2): 17.0.3
Operating system and version (eg, Ubuntu 17.04): server: raspbian GNU/Linux 10 (buster), client Arch (up-to-date)
Apache or nginx version (eg, Apache 2.4.25): Apache2.4.38
PHP version (eg, 7.1): PHP Version 7.3.11-1
The issue you are facing:
When creating a new directory on my client, the Nextcloud client application says "Not allowed because you don’t have permission to add sub folder to that folder USB/dirA/dirB/tmp_test even though the directory USB/dirA & USB/dirA/dirB is set to permissions 777 on the server. The directory is located on external storage mount and which is mounted to /mnt/usb/. In the Nextcloud admin interface under External stirage, I have the following:
Folder name: USB
External storage; local
Authentication: None
Configuration: /mnt/usb
Available for: my user | admin(group)
Under Advanced settings:
Enable previews [checked]
Enable sharing [unchecked]
Check for changes : Once every direct access
Compatibiity with Mac NFD encoding (slow) [unchecked]
Read only [unchecked]
Is this the first time you’ve seen this error? (Y/N): Y (am a relatively new user)
Steps to replicate it:
- Create a new sub directory in Next cloud directory where external drive is mounted (on client)
~/Nextcloud/USB/dirA/dirB/tmp_test/ - Dir remains unsynced (verify on web interface & little icon that appears next to dir icon (in dolphin, plasma))
- Selecting "Force sync now’ in Nextcloud client, displays multiple errors that read “…/path/to/file: Not allwed because you don’t have permission to add subfolders to that folder”
The output of your Nextcloud log in Admin > Logging:
[updater] Info: \OC\Updater::resetLogLevel: Reset log level to Warning(2)
GET /nextcloud/core/ajax/update.php?requesttoken=etU3OQRetd0xJX3PIeNt%2BFhRiOSd0cjv1lfsKbVZoDk%3D%3ATrR%2Bb0M2z7dTfAf9Rdc8lAAnwdHcmvnEkyKkT%2FFymHI%3D
from 192.168.1.1 at 2020-02-02T18:01:18+00:00
[updater] Info: \OC\Updater::maintenanceDisabled: Turned off maintenance mode
GET /nextcloud/core/ajax/update.php?requesttoken=etU3OQRetd0xJX3PIeNt%2BFhRiOSd0cjv1lfsKbVZoDk%3D%3ATrR%2Bb0M2z7dTfAf9Rdc8lAAnwdHcmvnEkyKkT%2FFymHI%3D
from 192.168.1.1 at 2020-02-02T18:01:18+00:00
[updater] Info: \OC\Updater::updateEnd: Update successful
GET /nextcloud/core/ajax/update.php?requesttoken=etU3OQRetd0xJX3PIeNt%2BFhRiOSd0cjv1lfsKbVZoDk%3D%3ATrR%2Bb0M2z7dTfAf9Rdc8lAAnwdHcmvnEkyKkT%2FFymHI%3D
from 192.168.1.1 at 2020-02-02T18:01:18+00:00:
[updater] Info: \OC\Updater::finishedCheckCodeIntegrity: Finished code integrity check
GET /nextcloud/core/ajax/update.php?requesttoken=etU3OQRetd0xJX3PIeNt%2BFhRiOSd0cjv1lfsKbVZoDk%3D%3ATrR%2Bb0M2z7dTfAf9Rdc8lAAnwdHcmvnEkyKkT%2FFymHI%3D
from 192.168.1.1 at 2020-02-02T18:01:18+00:00
The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):
$ sudo cat /var/www/html/nextcloud/config/config.php
<?php
$CONFIG = array (
'instanceid' => 'ocqh8k0dgbes',
'passwordsalt' => '[secret]',
'secret' => '[secret]',
'trusted_domains' =>
array (
0 => 'raspi',
1 => '192.168.1.198',
2 => 'my.url.org',
),
'datadirectory' => '/var/nextcloud/data',
'dbtype' => 'sqlite3',
'version' => '17.0.3.1',
'overwrite.cli.url' => 'http://raspi/nextcloud',
'installed' => true,
'maintenance' => false,
'updater.secret' => '[secret]',
'theme' => '',
'loglevel' => 2,
'mail_from_address' => '[secret]',
'mail_smtpmode' => 'smtp',
'mail_sendmailmode' => 'smtp',
'mail_domain' => '[secret].com',
'mail_smtpauthtype' => 'LOGIN',
'mail_smtpauth' => 1,
'mail_smtphost' => 'smtp.[secret].com',
'mail_smtpport' => '465',
'mail_smtpsecure' => 'ssl',
'mail_smtpname' => '[secret]',
'mail_smtppassword' => '[secret]',
);
The output of your Apache/nginx/system log in /var/log/____:
$ tail /var/log/apache2/access.log
192.168.1.1 - - [02/Feb/2020:19:06:43 +0000] "GET /nextcloud/index.php/apps/logreader/poll?lastReqId=Yxhn7Yjm7DybTiR47MUU HTTP/1.1" 200 757 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:72.0) Gecko/20100101 Firefox/72.0"
192.168.1.1 - stdcerr [02/Feb/2020:19:07:13 +0000] "PROPFIND /nextcloud/remote.php/dav/files/stdcerr/ HTTP/1.1" 207 4935 "-" "Mozilla/5.0 (Linux) mirall/2.6.2git (Nextcloud)"
192.168.1.1 - stdcerr [02/Feb/2020:19:07:10 +0000] "PROPFIND /nextcloud/remote.php/dav/files/stdcerr/ HTTP/1.1" 207 4920 "-" "Mozilla/5.0 (Windows) mirall/2.6.2stable-Win64 (build 20191224) (Nextcloud)"
192.168.1.1 - stdcerr [02/Feb/2020:19:07:16 +0000] "PROPFIND /nextcloud/remote.php/dav/files/stdcerr/ HTTP/1.1" 207 4919 "-" "Mozilla/5.0 (Linux) mirall/2.6.2git (Nextcloud)"
50.68.120.32 - stdcerr [02/Feb/2020:19:07:16 +0000] "PROPFIND /nextcloud/remote.php/dav/files/stdcerr/ HTTP/1.1" 207 4903 "-" "Mozilla/5.0 (Linux) mirall/2.6.2git (Nextcloud)"
50.68.120.32 - stdcerr [02/Feb/2020:19:07:18 +0000] "PROPFIND /nextcloud/remote.php/dav/files/stdcerr/ HTTP/1.1" 207 1166 "-" "Mozilla/5.0 (Linux) mirall/2.6.2git (Nextcloud)"
192.168.1.1 - stdcerr [02/Feb/2020:19:07:20 +0000] "PROPFIND /nextcloud/remote.php/dav/files/stdcerr/ HTTP/1.1" 207 4920 "-" "Mozilla/5.0 (Linux) mirall/2.6.2git (Nextcloud)"
192.168.1.1 - - [02/Feb/2020:19:07:21 +0000] "GET /nextcloud/ocs/v2.php/apps/notifications/api/v2/notifications HTTP/1.1" 200 1430 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:72.0) Gecko/20100101 Firefox/72.0"
192.168.1.1 - - [02/Feb/2020:19:07:05 +0000] "GET /nextcloud/index.php/apps/logreader/poll?lastReqId=Yxhn7Yjm7DybTiR47MUU HTTP/1.1" 200 757 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:72.0) Gecko/20100101 Firefox/72.0"
192.168.1.1 - stdcerr [02/Feb/2020:19:07:26 +0000] "PROPFIND /nextcloud/remote.php/dav/files/stdcerr/ HTTP/1.1" 207 4919 "-" "Mozilla/5.0 (Windows) mirall/2.6.2stable-Win64 (build 20191224) (Nextcloud)"