My first thought is you are trying to move the log file, not just copy it.
Another is: I have had this issue, but when trying to save an edited .cfg file. It seems Windows "SmartScreen" is latching on to certain files for some reason I can't fathom, so, security related, I guess.
My work around is to open aforementioned file, do my edit, open Task Manager, kill the Smart screen process, then save the file. I haven't found out how to kill smart screen for good. I'm sure there is a way, but, well, let's face it, I'm kinda lazy on the weekends.
Maybe this is why you haven't been able to move a log. But I don't see why you couldn't copy a log, though.
EDIT: THIS link will explain how to turn Smart Screen off.