Can't login into my Gitlab account via Firefox (error 422)

It is amazing how many posts have gitlab forums about this issue and most of them are not solved.
It seems like in my case this only happens on Firefox.
I tried to login via username, email, OAuth, nothing works on Firefox.


Thanks in advance.

Does it work on other browsers? Do you have any extension installed?

What version of Firefox? What OS are you using?
Have you tried going to https://gitlab.com/ directly instead of the “go back” link in the screenshot? (you might already be logged into an account via one method and you’re trying to log in with a different account)

I have exactly the same problem form many months for personal Gitlab AND Gitlab.com.

OS: Arch
Browsers: Firefox for Arch and Firefox developer edition (with any extensions)

From what I could see during the login process a strange cookie is returned by the server:

date:    Thu, 24 Sep 2020 09:50:46 GMT
expires:    Fri, 01 Jan 1990 00:00:00 GMT

when I switch to my Windows everything work (personal Gitlab and Gitlab.com).

I have the same issue. Firefox 81.0.1. Artix Linux (same Arch).