/InnovatorServer/NotificationServer/UserNotifications/ProxyPage.aspx needs more than 20 seconds

Hi,

I'm building up an environment with ARAS 11SP15 behind a Reverse Proxy via Port 9443, but I have serious problems... login is possible, the mnu is visible, and I can see some data... but when I try to create a user or part I only see a blank screen and the wait-for-me icon but nothing happens. 

In the logs I found a /InnovatorServer/NotificationServer/UserNotifications/ProxyPage.aspx with a response code 200 but slighly more than 20 seconds response time.

I think, ARAS tries to connect something but the server or port is wrong or firewall blocks the traffic. What happens in the method? Is there some kind of communication and where can I configure it?

Thank you,