Part of the mystery solved.... When I try with Firefox, at least I can see an error message under the login that means something. For instance "incorrect password" - that's not showing up in Safari.

But even with Firefox, Google has come back to tell me that I don't have an adsense account with a particular email address. Even though I've received a message from Google TODAY to that email address about AdSense (the contents confirmed it was the adsense account - and it wasn't a phishing email).