I've not used that action, but I would recommend creating a variable with a default value of a path that you know works (as in, if you were to just type the path into the Copy to File Share workflow Action, it works). 

If the var with the same string works in place, then you know variables can work correctly and the way you're setting the variable data is likely the the culprit. 

However if your direct string works, but a variable of the same exact string doesn't, then you have your answer that something strange is going on (it wouldn't make sense that a variable couldn't be used in an action and would indicate maybe something is wrong with the version of Nintex WF that you're using and there's an update, or there's an actual bug that needs fixing and you should open a ticket with Nintex Support proper!)

Let me know the results and we can go from there!