Synchronization problem, split brain?

Nextcloud version (eg, 20.0.5): 20.0.8 at the momemnt issue happened on previous versions too
Operating system and version (eg, Ubuntu 20.04): Debian 10
Apache or nginx version (eg, Apache 2.4.25): Apache/2.4.38
PHP version (eg, 7.4): 7.4.15

The issue you are facing:

User A files dissapear from local folder after few seconds. Folder, to which the files ( .pdf .txt .gpc) are copied to is owned by a different user B and is shared to user A. User A is using his laptop with Win10 and user B is also active at the same time on Windows server 2012 R2. In the activity log of user B there is a information that the file was succesfully synchronized meaning deleted from server to trashbin. User A wanted to provide me with a video of the error so he tried reproducing it. The problem happened partialy when the file got deleted from his PC for a moment and then synchronized back from the server after a second

Is this the first time you’ve seen this error? (Y/N): No

Steps to replicate it:

  1. Setup NC server and 2 clients A and B
  2. Upload 55 000 files, 80GB to client B
  3. Share a folder to client A
  4. Download or copy files to the shared folder on user A computer

The output of your Nextcloud log in Admin > Logging:

I can send the logs if its necessary, no errors concerning synchronization. 

The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):

$CONFIG = array (
  'instanceid' => 'xxx',
  'passwordsalt' => 'xxx',
  'secret' => 'xxx',
  'trusted_domains' =>.
  array (
    0 => '',
  'datadirectory' => '/srv/nextcloud/data',
  'trashbin_retention_obligation' => '30,90',
  'dbtype' => 'mysql',
  'version' => '',
  'overwrite.cli.url' => '',
  'dbname' => 'nextcloud',
  'dbhost' => 'localhost:3306',
  'dbport' => '',
  'dbtableprefix' => 'oc_',
  'dbuser' => 'oc_xxx',
  'dbpassword' => 'xxx',
  'installed' => true,
  'mysql.utf8mb4' => true,
  'maintenance' => false,
  'ldapIgnoreNamingRules' => false,
  'ldapProviderFactory' => 'OCA\\User_LDAP\\LDAPProviderFactory',
  'theme' => '',
  'loglevel' => 0,
 'memcache.distributed' => '\\OC\\Memcache\\Redis',
 'redis' =>.
  array (
    'host' => '',
    'port' => 6379,
  'memcache.locking' => '\\OC\\Memcache\\Redis',
  '' => 'stable',

The output of your Apache/nginx/system log in /var/log/____:

no errors