Hi V22,
I am suspecting that your error might actually not be related to the serial at all, based on what you said -- it sounds like you might be having a "serialization error," which is .NET's way of saying that it cannot convert some data from text to binary. In this particular case, that would mean if your settings.dat file was corrupted somehow. If that is indeed the problem, simply removing or renaming your settings.dat would fix it up (but then you'd have to re-enter your profile information, license key, settings, and re-win offline achievements and high scores -- savegames are unaffected, so simply loading up old saves fixes it). Settings.dat is either located in the game folder, or under C:\Users\yourname\AppData\etc. If it is giving you an error on load, it should also be giving you a directory where the error details are logged. If you go to that directory, you'll be able to also find the settings.dat. Hope that helps!