I managed to get on a report page via the KDP portal. Very weird, since it's the same URL, but the form showed up because I was coming from a different referred. Oh well, reported successuflly now (I think).

Thx for confirming it wasn't just me. I was starting to doubt so I edited my comment.

update: I figured out what is going on... It's the classic curse of the extra slash!

- BROKEN = https://www.amazon.com/report/infringement/

- WORKS = https://www.amazon.com/report/infringement