did you get that window/message from main site log in, or from FB site log in?
it's from fb log in
In my pc I can log from Opera & Mozila but not from chrome
From my work's pc I can't log at all.
My other account can log to all.
From my mobile ok.
After trying some things : running Malwarbytes, reinstall Chrome, turn back 2 days the clock, problem still there.
It seems that I must download kasperski (that's what fb recommend) and run it to get my fb back.
I believe it's a classic b@s@, run the "free" program, suppose to clean my pc but to be secure better download the premium version blah blah ...