I’ve just discovered this - hope it helps anyone.
If you’re using Mozilla Firefox and have chosen that you use Google account for Google AdSense (a wizard which, at some point, becomes an endless loop - wizard finishes and begins from the beginning), while logged in at, for example, GMail - going at www.google.com/adsense causes (again) an endless loop because Google AdSense tries to log you in (because you’re logged with your Google account) but it kicks you back at login page where Google AdSense tries to log you in (because you’re logged with your Google account) but it kicks you back.. Ah, that stupid recursion! :)
The solution is, as I’m so ashamed to say, to use Internet Explorer. Finish the wizard with IE (for choosing your Google account for accessing AdSense) and that’s it. Now try with Firefox and all is OK.
I presume that the problem is that with Firefox, for some strange reason, the above mentioned wizard never actually finishes.
Best regards.