close notification goes to external email NOT customer

[MacHaensel]

I've a strange error in OTRS sending the close notification.
I'm using version 5.0.8

The process is as follows:
1. receive a ticket via email
2. send an email for request to a other email
3. the external send a response to the ticket
4. close the ticket via "close" button (notification is configured with event "TicketStateUpdate" and send to "customer of the ticket")

Everything is working fine, but the closing notification is send to the external email, NOT the customer of the ticket.
I tried it with several adresses and it is always the same. If the customer send a response as last one, the notification is send to him.
So I'm supposing OTRS changes the Customer email to the last one it gets a message from?!

To complete the whole confusion:
At my testsystem the notification is send to the customer. Like I want it.
I've imported the Sysconfig from the live system to the testsystem and it still works. So I don't know what to do to make it work.

Does anybody know a solution for that?

Kind regards
Hans