I am travelling at the moment and not at my base. I am on Win7 and I dont have a problem (or else I would have fixed it already).
I am guessing that in the case of the few people who have problems with the add-on, there is something that is interfering with it. With WebExtesions, there is no need to restart the browser.
You would get a pop up error message if there was a failure to save to Clipboard, and since I presume you are not getting it, something else is interfering.
I will try to find out the reason once I get back.