@girish
I tried this exact same setup. I was able to connect to the storage via SFTP and my SSH key and I copied my private SSH key as instructed to Cloudron.
When I now try to save the backup settings using SSHFS it keeps working and never finishes. The log says
Jan 14 11:40:33 box:shell addMount spawn: /usr/bin/sudo -S /home/yellowtent/box/src/scripts/addmount.sh [Unit]\nDescription=backup\n\nRequires=unbound.service\nAfter=unbound.service\nBefore=docker.service\n\n\n[Mount]\nWhat=uxxxxx@uxxxxx.your-storagebox.de:/\nWhere=/mnt/cloudronbackup\nOptions=allow_other,port=23,IdentityFile=/home/yellowtent/platformdata/sshfs/id_rsa_uxxxxx.your-storagebox.de,StrictHostKeyChecking=no,reconnect\nType=fuse.sshfs\n\n[Install]\nWantedBy=multi-user.target\n\n 10
Jan 14 11:40:44 box:shell addMount (stdout): Failed to mount
Jan 14 11:40:44 box:shell addMount code: 3, signal: null
Jan 14 11:42:04 box:shell removeMount spawn: /usr/bin/sudo -S /home/yellowtent/box/src/scripts/rmmount.sh /mnt/cloudronbackup
Not sure what else I can try.
Edit: magically and waiting one night it works now.