KB 4103721 Problems, here is a solution

It actually worked for me. I still have problems with Defender, but that's nothing after the system crash. My notebook supplier (Schenker-Technologie in Leipzig) is also reporting this to Microsoft. I don't know if my feedback hub worked. I assume Microsoft will offer a solution in the next few days. Otherwise, I'll simply reinstall version 1803, this time accepting everything in the data protection settings and only then disabling everything. Immediately afterward, I'll hide the update.
Thank you for your feedback; the whole process was actually a bit of fun. Until the next Microsoft debacle.