I run a number of different instance of MFP at the same time, and this morning some logged on fine and some came up with the web server error. Maybe shutdown the software and try again, as that is what I did with the versions that had the error for me.
I don't know if the error is just to do with the splash screen we have when there are no markets loaded. Possibly that is what was down, as the betfair API status page showed no errors https://status.developer.betfair.com/