A few days ago this works without problems, right now I need a "AVV"
You probably did some changesd to the question, and by saving the question you applied the new rules that habe been in effect for several weeks by now.
The point is: YOU need the DPA (ger. AVV) to comply with GDPR (DSGVO), article 28, to be more concise. If you do not have a DPA and process email addresses (which are person-related data), then you most likely violate the GSPR which is not exactly a good idea.
As we have observed that many of our users are not aware of their legal obligations, we added a function to make sure that they won't miss them. This applies to all new question of the types "opt-in" and "contact data", and those that are edited. The online DPA does not involve additional costs, so it's probably a good idea to take care of that soon. For details see Personal Data. For even more background details, please contact the data protection officer of your institution.