From what I can tell, the ad appears to be region-specific and is intermittent.
So far I have not seen any pop-up ads here in Ireland or while I was in Germany.
However, after trying a VPN connection to simulate being in the USA, an ad appeared and I was already logged in:
It did take a handful of VPN connection attempts to trigger the ad, so whoever is behind these dodgy ads seem to blocking VPN IP addresses/ranges similar to what content providers to enforce their region restrictions.
When I get time, I'll try doing a trace to see what's triggering it.