I've resolved, the problem was on my side.

I had blocked the http-referer for extra anti-tracking, but that apparently blocks the login form from working.
I've re-enabled the http-referer and now the login form works again.

There are only few sites which break by lack of http-referer, and most often it's the sites which try to fingerprint-datamine more, endchan just accidentally is among those when it comes only to the login form. The rest of endchan works fine even without http-referer.