After fighting with the same issue for about a month I managed to fix it by downgrading to from 2.5.33 to 2.5.31. The problem may also be related to the docker releases, because like a couple of other people in this thread I was using urbackup in docker on unraid, but in my case it was the uroni/urbackup-server image, not the binhex one.
I now moved to the binhex/urbackup image because the official image doesn’t list indivdual patch versions, so it’s not possible for me to roll back to an older patch version. I really suggest considering to listing patch versions as individual images in addition to the current 2.5.x and latest ones on dockerhub. Offering (exclusively) unstable images kind of defeats the purpose of having an immutable docker environment.