Self service debugging

I am trying to set up the self service according to https://www.univention.com/blog-en/2019/04/ucs-4-4-self-services-new-features/ on a UCS 5. I see strange error messages

Screenshot_20210920_220418

Although, in this case, the user definitely has a primary e-mail address. Is there some current documentation how to debug this step by step? I only see older docs from 4.x UCS …