Directory is unavailable - when external sotrage is mounted

Hey Folks,

I’ve a new installed Nextcloud (uptodate) on a DietPi. It works fine. But when some external storage is “mounted” the system shows “This directory is unavailable, please check the logs or contact the administrator” when some storages are checked fully. I’ve tried to mount as FTP, CIFS, LOKAL → always the same issue after a fiew hours (after the intial check of the external sotrage done by Nextcloud.) The Error-Message only appears on accounts with access to the specific external storrage.
I think it is becaus of german umlauts (or something like that). I didnt have this issues on Nextcloud before (my last Installation is abaout 2 or 3 years ago.)
Any Ideas?