Hi Jeroen!
@pfaffman called it, this turned out to be a core PR that accidentally broke subfolder setups like yours (ie /forums), but we have since reverted it. The revert is already in the tests-passed branch, so if you update from the admin UI or do launcher rebuild, you’ll get the fix.
Apologies for the issues and thanks all for raising this! Seeing that this site used subfolder was helpful in pinpointing the cause.