I had problems settings this up from the beginning of Unifi in my network. I now got the solution to that.
You must not assign a VLAN to user which is already assigned to a WLAN Profile. Because in the UAP for every VLAN is an actual virtual interface on the WLAN NIC created, and you can't create two on one!
Please advise to create an WPA Personal so MAC spoofing is not in issue