Thank you so much to all three of you.
@mro
It was not because of the alias. My guess: You are allowed to write it wrong, if it is not used as resource. From you I have picked up the tool jq. My hope: With this tool I will find such Copy&Past errors in the future.
@nightpool @macgirvin
It was due to the wrong content type. After I changed it, my profile http://dimna.de/user/astrid/ was found in Mastodon instances.
I felt safe with my wrong Content-Type, because my handel @astrid@dimna.de was found. Now I did some research and found here that Webfinger is probably not that strict: Problems POSTing to Mastodon inbox - #14 by nightpool
I haven’t quite understood that yet. But now that it works, I’m curious to see how I get on.