This has been mentioned now by a few people and it does not ring well in my ears.
It is true that there's not too much we can do about it at this point, it must be an ad served by googles adsense so there's not really anything that can be done to filter a certain ad like that as far as I know. I don't even see how an ad like this would be allowed by googles guidelines to begin with unless they changed something since it covers up the whole site and screen of the device. Hopefully this ad will get removed but no one can really tell at this point.
The suggestion of trying another browser might be worth a shot for now, but also did they other login box change anything for you? Instead of using the login displayed above the index, click the "login" tab first without writing in any information and it will take you to it's own login page, maybe this ad is not displayed in the same manner at let's you log in unless that's what you've been trying to use all along.