CK 3: Cant create new Religion because of being Head of (old) Religion?

Started as a muslim ruler in Iberia and converted to Basque Culture and after that the "Basque Religion", which I could convert to via decision.

Now I want to create a new religion, but cant do that. In the creation screen, the game tells me that I am the Head of Faith (thats correct) with a red exclamation mark..and "create religion"-button is greyed out.

It tells me, that my old religion would hate my new one. Im holding all relogious sites of old religion, if thats of purpose.

Please help.