Try deleting the file 'Game.prefs' which is located here:
YOUR USERNAME\My Documents\My Games\Gas Powered Games\Demigod
Then close Impulse and run the following file:
http://sd.stardock.com/curthendzell/fix_registrations.reg
Make sure to follow the link in Internet Explorer, as other web browsers will not handle the file type correctly. Once you follow the link, save the file and double click it to apply it. Then relaunch Impulse. You will need to re-login with your username/e-mail (either will work) and password. Then launch Demigod and try to log into multiplayer.
If none of that helps, then drop us an email at [email protected], and we'll try to get that straightened out for you.