OK what I did was first I created a new password then I found the save game folder and unchecked the setting thats marked as read only for that folder..windows prompts apply to all sunfolders etc in that folder etc..click yes.
Once you enter your key on the MP screen it should now stay saved...I've just run 3 matches on MP (got hammered lol) and didn't need to re enter key at all.
worked for me so try it if nothing else works.

PS..I entered the key in all lowercase no dashes also..