Synchronisation issues, inexplicable timeouts

Nextcloud version (eg, 20.0.5): 28.0.4
Operating system and version: I use shared hosting at all-inkl in Germany
Apache or nginx version : I can only confirm that they use nginx
PHP version (eg, 7.4): 8.1.27
Windows client: 3.12.0

The issue: I’ve been using Nextcloud for a few years without any issues. The last couple months I am struggling with syncing any updates to my cloud via the Windows client at all. My Windows client starts syncing, and then just freezes, and I get a notification that there was a network timeout. When I restart the client, it starts syncing again, creates a few placeholder files, and then eventually freezes again with the error that the connection has timed out. After reconnecting, the system tray error shows undefined state and it perpetually shows that it’s waiting to synchronize.

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

Steps to replicate it:

  1. Start Nextcloud Windows client
  2. Sync

The output of your Nextcloud log in Admin > Logging:

Server logfile actually shows nothing, Windows logfile shows timeout

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

$CONFIG = array (
  'instanceid' => 'XXXXX',
  'passwordsalt' => 'XXXXX',
  'secret' => 'XXXXX',
  'trusted_domains' => 
  array (
    0 => 'XXXXX',
    1 => 'XXXXX',
  'datadirectory' => 'XXXXX',
  'dbtype' => 'mysql',
  'version' => '',
  'overwrite.cli.url' => 'XXXXX',
  'dbname' => 'XXXXX',
  'dbhost' => 'localhost',
  'dbport' => '',
  'dbtableprefix' => 'SGu5J_',
  'mysql.utf8mb4' => true,
  'dbuser' => 'XXXXX',
  'dbpassword' => 'XXXXX',
  'installed' => true,
  'tempdirectory' => 'XXXXX',
  'mail_smtpmode' => 'sendmail',
  'mail_sendmailmode' => 'pipe',
  'mail_from_address' => 'XXXXX',
  'mail_domain' => 'XXXXX',
  'default_phone_region' => 'DE',
  'maintenance' => false,
  'theme' => '',
  'loglevel' => 2,
  'log_rotate_size' => 104857600,
  'bulkupload.enabled' => false,
  'updater.secret' => 'XXXXX',

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

I have no access to the system log due to shared hosting.