VLAN not creatable

I have a bit brain lag. I have been asked to create a vlan interface on a 100A. It is using 4.2.9 internal is configured as switch interface -> :( but with lots of rules/objects in place a change now is not possible. The wanted to have the config of course on internal. this fails with the cryptic error message " Invalid Input" in Gui or on CLI: " FG-HA01 # config sys interface FG-HA01 (interface) # edit wifi_mobile new entry ' wifi_mobile' added FG-HA01 (wifi_mobile) # set interface internal FG-HA01 (wifi_mobile) # set vlanid 4 FG-HA01 (wifi_mobile) # set ip 10.200.41.254/24 FG-HA01 (wifi_mobile) # set vdom root (wifi_mobile) # end object set operator error, -651 discard the setting Command fail. Return code -651" i tried to create the VLAN on DMZ1 interface now it fails with either " invalid input" or " Ip Adress of interface is same as others" - while the other interfaces have a 10.200.254.254/24 (internal) and an official IP on wan1. Any hints what actually goes wrong there? I was one time (for a test able to create the vlan using a different IP e.g. 192.168.17.1/24) but after deletion of the vlan even that fails in consecutive attempts with invalid input