Full error:

mount error(115): Operation now in progress
Refer to the mount.cifs(8) manual page (e.g. man mount.cifs)

This could happen if it tries to mount via dns name and it resolves to IPv6 e.g. example from fstab:

#//u111111.your-storagebox.de/backup /mnt/backup-storage cifs iocharset=utf8,rw,credentials=login.txt,file_mode=0660,dir_mode=0660 0 0

Try pinging the u111111.your-storagebox.de to get the ipv4 address (ping -4 u111…) and enter that in /etc/fstab/ (example if ping resolves to 11.22.33.44):

//11.22.33.44/backup /mnt/backup-storage cifs iocharset=utf8,rw,credentials=login.txt,file_mode=0660,dir_mode=0660 0 0

then try

mount -a

• Use Hetzner storage box as docker volume