Is this error perhaps not a third DHCP, but a second DHCP on a physical interface? I wonder if that's not a limitation of the DHCP server at present - that only one VLAN on a physical port can have a DHCP server.
So, the Guest VLAN DHCP doesn't work even when it's the only DHCP server? What if you delete the VLANs, then set up just the Guest VLAN and create a DHCP server on it?
That is correct. But the case is solved. I feel a little embarassed. I have two switches in two locations connected via fiber. There were a little fault in VLAN-configuration of one of the switches. Once corrected it worked.
So i can confirm that it DOES work with two dhcp-servers running different VLAN's.