A few things I tried:
- Tried to create a new Kunena Category menu item, used that to load the forums and tried again, same result
I checked your menus. The sort of the menu items are not correct.
The menu item 1504 Forum linked to the defaultmenu=1505 and target is forum/index
The menu item 1513 Forum linked also to the defaultmenu=1505 but the targed is forumnl
You can't use sundry targets with the same menu ID.
But that's only a single example. You have more then one menu items with the same target, but different address.
You must repair the menu items, otherwise remain the problem.