Multiplayer standalone doesn't find session anymore. No changes to project or engine

When I test my project (multiple windows, standalone) and host (listen server, LAN) in one window, I can’t find the session in the others.
Now, it could be a mistake by me, right? But it worked before, always, reliably.

I continued with a copy of this project in a newer engine version and realized, that find sessions didn’t work anymore. So I went back to the older versions of the project, that definitely worked as I stopped with them.

The host is successful (I use advanced sessions plugin, the default unreal engine nodes don’t make a difference, I thried it), the find sessions is successful, too. There are no errors.

Strange to me: The standalone windows work with an export of this project over network to another computer (both server/client constellations), but the standalone windows don’t communicate among themselves on one computer (I also tried to test the project on another computer, same network. Same result).

So, I can export and use it, since I normally don’t want to play multiple instances on one computer, but I need to be able to test it in the engine for developing.

Because I’m not a programmer and at the moment not capable of c++, I don’t know, what’s going on and how to solve it, since this seems really strange to me because it worked before and I didn’t change anything. Actually I copied the project several times and none of these old projects find sessions anymore (UE 4.15, 4.17., 4.18, 4.19).

Thank you for your help,

I am having the exact same issue. Tried normal UE session, lan, no lan, set steamdevapid key etc… Did you figure it out? Also how did you get them to connect on different computers? Do you have to host the game somewhere first?