MF Pro's log-in window is based on IE's library, so these facts are related (the inability to log in via IE and the error message). We have two other users who experience similar problems (although they've never been able to log in to the newest version), and BetFair acknowledged that this is something at their end. However I'm not sure they will resolve this within a reasonable time.
As far as I understand, rebooting and reinstalling gives you nothing, doesn't it?