I have problem with one client, when file backup is working, server log showing message, example: "Error opening file… “/media/URBACKUP/client/…/file.jpg " retrying… Read-only file system (code: 30)” and backup failed.
Sorry for my bad English
Try looking at dmesg to find out what happend to your file system perhaps?
dmesg show problem with file system, i stop service urbackupsrv, umount partition and run e2fsck,
solved the problem