Oh right, trying with a different browser.
So it seems I CAN log in if I use IE, now I'd like to know why Firefox just decided to fuck up for this site (every other site from which I was logged off works fine).
I've been trying to log in with all extensions off so I don't think that's the cause (primary suspect was µblock origin) so I'm at a loss, I'm accepting all cookies from third parties until their expiration so that should not cause a problem.
"Remember me" is checked but since I can't log in in the first place it has no effect.
Really stuck on what's causing this, I didn't update FF or anything.