I tried Google-ing of course, but could not find anything. There were some posts regarding why / how some plug-ins for Rhythmbox keep popping windows. I did not find anything related to password window specifically. Still, I opened Edit->Plug-ins and observed that few plug-ins were enabled. For no apparent reason, I disabled the plug-in for Cover art (as shown below) and never got those disturbing pop up windows again!
What I don't understand though is, that plug in was always enabled afaik, because I never explicitly enabled / disabled it in past, then why suddenly after almost 2.5 years, it started popping up windows?! Another thing is, as shown in first image, there is something called "xml.amazon-proxy caching server" - Have no clue how it could be related to cover art of an album!
Anyways, will definitely post the explanation if I can find one, for time being let me enjoy the pop-up_window_free Rhythmbox!
