It's been a few days since my Hangout chat sometimes appears invisible (I mean, almost totally invisible):
The blank part was my edition, but the 'phantom' Hangout on the bottom right corner of the screen is the problem.
It's strange because it happens in different computers (at home, at work) and it seems to be related to when I click to open Hangouts where someone sent me a message while I was offline . In fact, it happens always to the first Hangout I click to open.
To fix it, I have to reload the page, open another Hangout and only then reopen the problematic chat.
Best Answer
The problem disappeared after a couple of days. I guess it was a minor bug.