Ok, there IS A SERIOUS ISSUE:

https://www.amerikaans-modelspoor.info/website /forum/

but also

https://www.amerikaans-modelspoor.info/website /community/belangrijk/ (for example)

I guess the second is correct. So you must rename the "forum" slugs.

OR decide what of two you want to use, and turn ALL to that (menu included). For now it is mixed.