I don't know when that happened, but I can't follow any game or series anymore. The Follow Button is just not there anymore on any game/ series page for me. Also the unfollow button for games I already follow isn't there anymore as well. Has anyone else that issue ? I'm using Firefox by the way.

Following series is not something that is implemented yet so it makes sense you don't see it there. As far as I'm aware nothing related to that button has changed recently though, and I can't replicate your issue. You're not by any chance using new extensions, or using the beta/nightly build of firefox?

Thanks to your suggestions I found the cause of this:

I did reinstall Firefox around a week ago with the only difference that I used adblock edge before and now installed adblock plus, which actually removed the follow button. So yeah deactivating adblock solves the problem.

Be sure to disable Adblock on this site. The ads are completely unintrusive and you'll only see them when your login expires.