Create/Join Session not working for Teammate in editor only?

Hi there.

First if all:
I‘m using UE4.21, Advanced Sessions for 4.21 and no additional Plugins. Steam is setup in the Ini, AppID 480 selected, Port is okay, Steam is running.

I followed this Tutorial:

When trying to test the CreateSession and JoinSession, my Teammate and me are stumbling over a Problem, which i think is a Problem with the Editor.

When we both try to connect while playinf in Standalone Mode via the Editor, i can Create a session without a Problem. But my Teammate can‘t join it - it always fails. Same if he tries to Create a Session.
It also fails when using unreals native CreateSession/JoinSession.

We tried to package the Game ias development build. And see there - Everything works - for both of us!!
Now i am confused, since the Problem seems only to appear on the Side of my Teammate, that he can‘t Create/Join via Editor Standalone.

Here a small overview:

Any idea, why this is not working via Editor?