MyBB Community Forums

In order to test things I have a few normal user accounts besides my admin account.

When I try to login in the Popup I enter the CORRECT user and password but still, I get logged in as another previous user. I have gotten a 15 minutes block so many times now that I need to find out if this is a cookie issue or what. 

Why am I getting logged in as another user than the one I specify in the login popup?

What's your URL? Either you have full page caching enabled, or your browser just isn't clearing cookies properly.

https://worldtrendsforum.org/

Might very likely be the cache but when I clear it It happens again after some time. Probably when both users are 'recorded' by the browser. But I find it weird that the popup contains one user but another is logged in...

Popup and main form are 2 forms, so the browser saves 2 entries.
This is a browser issue, not a MyBB one

It sounds like it might just be a browser autocomplete issue, or server caching. If you enter credentials of user A, you won't get logged in as user B.

I looked at the page headers, and saw there's some system called "Gigahost App Router 4" that might be caching pages. But it also says you're using PHP 5.2.17 - you might want to ask your host why you're on a server running a version of PHP released over 11 years ago (5.2 was first released 15 years ago).