I have installed UrBackup server on a Windows machine and several clients on Linux CentOS 7.
The backups are made successfully from Linux to Windows, I even get the progress bar in the web status window however no backups are shown (though the backup files exist on the Window UrBackup server machine).
First error is usual for Linux. What is your file system? Are you using LVM? Because you need to have free space for an extra LVM to create snapshots. Weird that it uses snapshots for file backups, though.
Other behavior is as expected, those files should be considered invalid and should have been removed when cleanup window comes around, or you can just run a command for it instead.
Can’t really say what’s wrong with your backup though, sorry. You could try to disable snapshots and do only file backups but I have no idea if you can simply disable this, maybe you will have to reinstall the client.