Okay, i have check this from first point.
MySQL is the problem. Not ISPConfig or phpmyadmin.
On Console i log in MySQL with "mysql -u web1_u1 -p". After succesful login i type "show databases;" an i see web1_db1. Now i enter "create database `web1=db1`;" like phpmyadmin do it and give another "show databases;".
I was shocked.
I can see the created database.
I think the only way to solve the problem is to remove the "_" from username. This char can replaces with any other special char.
Greets from Germany