I worked further on the issue: The 400 - Bad Request error only occurs with certain names for fields. For me specifically it occurred when I named a collection by the name “Police” which is the german word for “Policy” haha. Strapi dosen’t like endpoint names like that. Is there some kind of blacklisting happening or what’s going on there? Or maybe it has to do with automatic plural generation? I really don’t know what’s going on. If I name my collection “Policen”, the german plural, it works fine.
EDIT: Ahhhhhh the payload of the respone contains the following:
“Content Type name Police cannot be pluralized. \nSuggestion: add Item after the name (e.g News → NewsItem).”
Then it really was the pluralization 