Nevermind…
I think I have managed to get this working now. I generated new forms and then it started to work - including with my custom code. Maybe it was the custom PostURL that messed up the whole thing? 
Lawful basis doesn’t seem to be registered though, but I made another forum thread for that here: Lawful basis - how to get it working? to continue that topic in a separate thread.
Kind regards
PowerQuest