Looks like a permission problem:
Nov 30 16:07:31 Vault smbd[43571]: [2022/11/30 16:07:31.580337, 0] ../../source3/smbd/smb2_service.c:168(chdir_current_service)
Nov 30 16:07:31 Vault smbd[43571]: chdir_current_service: vfs_ChDir(/mnt/user/backups) failed: Permission denied. Current token: uid=1000, gid=100, 4 groups: 100 3003 3004 3005
Show the output of this command:
ls -la /mnt/user/