It might take you to take a look. Interrupted system call is a specific thing that you could chase with strace (I think) to see if you can see what sort of interrupt is happening, and what system call is suffering. I think there are few system calls that aren’t done by mono, so this might be a mono bug…
You could also possibly confirm that the issue is purely in FileEnumerationProcess by trying to avoid actual backup in various ways. An easy one is to do a test backup on a small file set without change. Similar testing on non-Samba area would confirm whether or not the problem only occurs on Samba.