For some reason the forum permissions were totally messed up.
Custom user groups like "Dutch", "Moderating", "Global Moderator", 'TMS SA-MP Admin",... couldn't see all boards they were supposed to see.
This is why I did a complete re-check of the forum permissions, which took a proper amount of time. (There wasn't a proper way to reset this, so I had to manually edit all user groups in every single board)
Fortunately all permissions are now like they're supposed to be.
A small summary:
Custom user groups like "Dutch", "Moderating", "Global Moderator", 'TMS SA-MP Admin",... couldn't see all boards they were supposed to see.

This is why I did a complete re-check of the forum permissions, which took a proper amount of time. (There wasn't a proper way to reset this, so I had to manually edit all user groups in every single board)
Fortunately all permissions are now like they're supposed to be.
A small summary:
- Guests and registered members see all the normal boards.
- Members in the "Dutch" group have access to "The Dutch Board".
- Members in the "TMS Server Admin" group have access to "Server Admins" board and are the only members that can post in the "Development" board.
- Members in the "High Society" group or members with at least 1000 posts have access to the "High society lounge" board and children.
- Members in the "Moderating" or "Global Moderator" group have access to the "Moderators" board.
- Administrators have access to deprecated boards.