So you know that I love Veeam FastSCP. Even the free Version is pretty cool. So I had an instance where I could not copy files from my ESXi server datastore1 to datastore2. I know the rights were OK, I mean I was root. Having turned on SSH I could Putty in and create folders at will.
The answer is check your version. I had 3.0, download 3.1 and it worked like a charm.