Now a login screen (like this) pops up:
Whatever I fill in, it doesn't connect to the database server. I get the following pop-up:
The application works further fine. I only cannot print any report.
SQL 2017 express on a DC with windows server 2016
Best answer by Marco Kleiweg
Probably is it not a good idea to place SQL on a Domain Controller. I was confronted with it. Next time don’t look for answers, but switch SQL to an other server.
Anyway: problem solved