Sorry you are having issues. To solve the problem the client (debug) log would be useful.
If possible, could you post it or send it?
This post describes how to change the client 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
If it is in the CBT info screen, copy & paste the complete message here please.
Thanks!
W.r.t. to backup size. CBT doesn’t really influence that. It just makes backups faster, since it doesn’t have to look at the whole file content/image content and can also give you a better progress bar.
Identical files should not be loaded if you have client-side hashes enabled, run the clients as Internet clients and use a file system on the server where it can hard link (all setups except the ZFS snapshotting one).