4.27 Blueprints Session Errors via Steam

Hi there. I started developing a game for steam with 4.26 and everything worked fine. I’m using Blueprints, the built in “Online Subsystem Steam” and the normal (not advanced) Sessions. But since the upgrade to 4.27 I can’t host a Online Session via Steam anymore. It doesn’t even work when I upload my new Builds to Steam (I have my own AppID).
When I use LAN I can still create a session. And my second PC can still join. But if I install the build via Steam the joining PC will time out.
I’m quite confused because everything worked fine with 4.26. Can anyone help?