It works now.
Okay, you can work around this problem if you reset the administration password. Next you will get an email with a new one, it just took a while for me to receive it. So this issue can be solved by more documentation and doing a reset directly after creating the mailinglist. I believe it is okay that the unser passwort is not directly accepted for now. This probably would be a major change.
This still is a problem today.
No attached documents