To solve the problem the server (debug) log would be useful.
If possible, could you post it or send it?
This post describes how to change the server to debug logging, where it is stored and where to send it to if posting is not possible: Having problems with UrBackup? Please read before posting
You mentioned you server version (2.2.8). That version isn’t supported… There were changes in 2.3.x which fixed issues with VHD backup chains. So it would also be important to know if you are using VHDs, what kind of chains you are using (incremental to last; incremental to last full?)