Thanks for letting me know. I never heard of LLDP before but read into it. Do I understand that right that it shouldn't have anything to do with it? Great that you found out how to make it work...

I tried some more stuff and realized that the error message shows up when I try to create more than one rule for the same port. So the error message is wrong. But that brings me to the same question...

If I want to create more than one rule it says "Only one DMZ rule is allowed for one WAN port." despite the DMZ option not being selected. Also why is the Source IP option limited to 5 entries?