Thanks Fabian,
but just now I found the cause, I think.
When I disable NinjaFireWall it works.
Now I have to find out how I can whitelist matomo in the firewall… the next support request there… 
Edit for those who have similar problems with Ninja/matomo combination: I found the solution, building a special file, on the support page of the Ninja Plugin: Admin blocked by NF!? | WordPress.org