I just tried to log onto my Google Adsense account using my Firefox browser and freaked out when I kept getting the "UsernamePasswdNotMatch" text on a blank white page.
I thought I'd typed in my password wrong -- or that Google was merging accounts or something worse!
Thank God I read thru some help screen that said to try a different browser and clear your cache before sending Google Adsense support an email.
I did just that -- I popped over to Internet Explorer, logged onto my Google Adsense account from there and lo and behold great God almighty, the "UsernamePasswdNotMatch" error did not appear, just the total of the lovely money I need to pay these bills and tithes and offerings and then some!
So if you found this post because you also got the "UsernamePasswdNotMatch" from the Big G -- don't fret, pet -- it might just be your browser.