Thanks for the answers, people.

Messenger.com is now working properly. I did nothing, it solved by itself.

By checking the new standalone Messenger for Facebook - aka www.messenger.com - one is unable to use it, as there's no option to log in using facebook credentials, and it doesn't matter whether or not you're logged into facebook.

I'm using Windows 8.1 64 bit and the following opera 28.0.1750.51 and these extensions:

LastPass
µBlock

Does this happend to anyone else and know how to solve it? Thanks.