I’m having the same problem, the error is popping up infrequently during a full file backup. URBackup is installed on an Ubuntu 20.04 LTS system, and the backup reporistory is an NFS-mounted share pointing to a Synology unit. There’s plenty of free space, and full file permissions as far as I can see.
Edit: Here’s a topic from 2016 discussing the same issue. I see another user like me who was using a share mounted to a Synology device, so I wonder if URBackup is getting caught up in some sort of nuance with network-mounted filesystems?