So, while I was lurking and doing a minor refreshing (going between New Pages and User CP), apparently the browser caused a 400 error page. Unlike other occasions, apparently the error was caused by a cookie, which as far as I can see was the cookie the system uses to authenticate an account (because of the info it has).

Erased the cookie, everything was fine. However, I decided to report it just in case someone else has the same problem, and also to tell Merlin to check that out just in case.

Waiting a reply. Also, "The More You Know..." (aka, if someone else had the same problem, post a pic with the page but with code info erased, and also explain what might be the problem).