Alright, so here is the usual mount.cifs output:
mount.cifs --verbose //***.your-storagebox.de/backup /mnt/pve/storage-box -o user=***,pass=***,vers=3.0
mount.cifs kernel mount options: ip=195.201.107.44,unc=\\u205813.your-storagebox.de\backup,vers=3.0,user=***,pass=********
mount error(2): No...