NAS backups fail with "stale file handle" on Fritzbox SMB shares (serverino mount option incompatible)

[jinxjinx3]

Environment

• umbrelOS running on Raspberry Pi 4
• Backup target: NAS via SMB (AVM FRITZ!Box 6690 Cable, FRITZ!OS 8.25)
• Storage on Fritzbox: exFAT-formatted external SSD

Problem
Setting up a Backup to a NAS (Settings > Backups > Another Umbrel or NAS) fails with:

Command failed with exit code 1: kopia repository create filesystem ...
ERROR: failed to open repository: unable to create format manager: unable to read format blob: error getting kopia.repository blob: error determining sharded path: error getting sharding parameters for storage: open /home/umbrel/umbrel/network/192.168.178.1/FRITZ.NAS/Backup/Umbrel Backup.backup/.shards: stale file handle
ERROR: unable to connect to repository: ...

Retrying afterwards instead gives "folder already exists", since the failed first attempt already created the target folder.

Root cause (confirmed)
The CIFS mount that umbrelOS creates for the NAS share uses the serverino option by default:

//192.168.178.1/FRITZ.NAS on /home/umbrel/umbrel/network/192.168.178.1/FRITZ.NAS type cifs (rw,relatime,vers=3.1.1,...,serverino,mapposix,reparse=nfs,...)

This is a well-documented incompatibility between the Linux CIFS client's serverino option and Fritzbox SMB servers, which return inconsistent/bogus inode numbers, causing "stale file handle" errors. Same root cause is documented for other projects (Home Assistant OS issue #2585, Proxmox forum threads, etc.).

I manually verified the fix: unmounting all umbrel-managed mount points for the share and remounting the same path manually with noserverino resolves the issue completely — I could write and repeatedly read files without any stale handle errors.

sudo mount -t cifs //192.168.178.1/FRITZ.NAS /home/umbrel/umbrel/network/192.168.178.1/FRITZ.NAS \
  -o credentials=/tmp/creds.txt,uid=1000,forceuid,gid=1000,forcegid,vers=3.1.1,noserverino,file_mode=0755,dir_mode=0755

However, umbrelOS appears to run a background process that automatically remounts the share with its default options (including serverino) within seconds of the manual mount being removed — so this isn't something a user can work around persistently from the outside.

Request
Could umbrelOS either:

1. Add noserverino to the default CIFS mount options for NAS backup targets (or make mount options configurable), or
2. Detect Fritzbox/AVM shares specifically and adjust mount options accordingly, or
3. At minimum, expose a way for users to customize CIFS mount options for NAS backup connections.

Given how common Fritzbox routers are in Germany/Europe as home NAS solutions, this likely affects a meaningful number of users trying to set up NAS backups.

Happy to provide further logs or test additional mount option combinations if helpful.