Ok thanks. It’s definately not a file permissions issue then. The only other thread I could find on the forum with your exact error is this, which unfortunately has no replies Nextcloud with 2FA sync
The title of the thread says they are using 2FA though? You’re not using 2FA by any chance are you?
Also, as another test, you could try using the web app at https://app.joplincloud.com/ and see if you still get the error syncing from there