Hi Martijn,
You were asking:"I think your right because the IP helper-address isn't available. When I connect to a router does the port on whitch the router is connect have to be in all the VLAN's ? Tagged or Untagged ?"
That depends. If you have multiple VLANs travelling over one physical link then ONLY ONE can be Untagged. However you are free to Tag them all. The port only needs to travel tohose VLANs up to the router that require Layer 3 functionality like DHCP Helper.
Hard to explain. Feel free to get back to me in case of further questions. Otherwise I can send you a sample topology.
Regards, Ardon