I found a very similar problem with another backup application called Restic (link to issue). That product is written using Go (not C# plus mono framework) but it suffered the same issue when trying to back up to a CIFS mounted directory.
The problem seems to be that the Linux kernel sometimes returns EINTR which is kind of a failure but not really. The calling application or framework is supposed to retry the operation.
The Restic team toyed around with updating their application to handle EINTR but then decided to wait for the underlying Go libraries to be improved and properly handle the situation.
So maybe this is actually a bug with mono in that it doesn’t handle EINTR correctly. Or maybe we could work around it in Duplicati somehow. I’m not really sure.
Anyway, after messing with this for several hours I have not yet found a workaround…