Deploy Web Client with Session Hosts in Sub Domain

Hello there!

Tha last couple of days I created a Lab-Environment with 1 DC, a Remotemanger with Gateway, Licencing and Broker Role installed and a Session Host. After a little Troubleshooting everything worked great.

After that I created a second DC and a Subdomain and a second SessionHost as well. The Second SessionHost (SessionHost2) joined the Subdomain. I managed to add the SessionHost2 to the Remotemanger in the rootdomain and installed the Sessionhost Role.

At that point, I'm pretty much stuck. My users from the sub domain can connect via Web Client without any problems, but I can't open any apps from my collection. I get this error pretty much constantly:

2019-02-08T09:40:41.703Z Connection(ERR): 
The connection generated an internal exception with disconnect code=ConnectionBroken(8), extended code=<null>, reason=WebSocket closed with code: 1006 reason: 
 Thrown in thread 396952 at:
    websockettransport.cpp(330)
Call Stack:
        at Rjb
        at Ojb
        at rp
        at Ugd

Now I tried pretty much everything I could think of. This all happens in the same Subnet with Windows Firewalls turned off.

Is this some kind of missing priviliges of the Subdomain-Users to authorize against the remotemanger in the root domain? Or is there a general Problem with a remotemanger in another domain as the session host?

For explanation: Iur company wants to offer hosted services in the future and wants to make sure, that different customers absolutely can't establish any connection to another company's hosts. If there is another way to acomplish that I would be very happy.

Thank you very much for your interest and input !