Thanks for the quick response!
We鈥檙e using the latest IFW REST-API features and Director 1.11 or master was a requirement.
I also read Discussion: Use the master branch or a tagged release? 
But of course you were right - I should have tested this in a released version.
Indeed it works in 1.10.2 
I am going to open a Github issue then.
Edit: someone was quicker than me Cant't create usergroup because of invalide property assign_filter 路 Issue #2784 路 Icinga/icingaweb2-module-director 路 GitHub
Sorry for all this noise!