@uroni’s patch fixed it for me too.
I’ve just installed OMV with URbackup on a rpi and have a similar problem. My backup folder is on a 1.78TB USB drive and the initial backup went for 7hrs before failing as described above.
Is there a viable solution that doesn’t require linux-type knowledge?
uroni’s patch has been already applied to the GIT repo (1 year ago!). Which version are you using?
I think you only need to update it