Some files not synchronized

Servus Community,

I host a Nextcloud Instanc on my own server and have three clients, a laptop and desktop (both Apple) and a smartphone (iOS). I had to upgrade the laptop to install the latest desktop client. That is now done and the desktop client has also synchronized everything except the encryption key of a Cryptomator vault. I see it in the web interface and on the desktop computer as well as on the iOS device but not on the laptop. Instead, I see a red X in the status bar on the laptop, which indicates a configuration error (e.g. a wrong URL), but what could be wrong if three out of hundreds of files are not synchronized?

After restarting the laptop it tries to synchronize the missing files and the icon in the status bar changes several times between green and red and in the Finder a yellow warning sign is displayed behind the Cryptomator directory. It also indicates an error in the encrypted data directories, here the activity log shows an error 503 Service unavailable on Get https://serverfqdn

What can I do to get the laptop client up to date or where could the error exist?

Thx & Bye Tom