OK theory is already destroyed. I can now see in which tab the field is that should have been conditionally hidden and is required if shown (I have made all fields un-required apart from this one, see picture).
I have checked literally everything now. Only thing I can think off is that the amount of fields is the problem. I have 192 fields with quite a big number (about 120) hidden due to conditioning on this one field "Type of Business".
I now had enough trying to find the error for today. I am making all fields un-required and the users can sort it themselves...
PS: This field "Health insurance" only shows up if Type of Business gets set to Dentist. It is on IT-specialist for the moment.