If their cases are like mine, which I believe it is, the bug is happening only with clients connecting to a dedicated standalone server, not PIE.
In my case everything runs fine in PIE when simulating a dedicated server and multiple clients. If I package my game and build the server.exe, things stop working ONLY IF I connect the client to the server.
I can launch the client as a stand alone game and it loads the default level fine. If I launch it connecting it to a localhost or remote dedicated server, the game session connects to the server, no error is given on the client or server logs, and the screen goes black after 1 second.
My main project was created using the precompiled version of the engine and my empty test project was built using 4.11.1 source and both have the same issue.
I’ve tested with a locally running server and a dedicated server hosted online and have ruled out networking (ports are all open, no firewall, I’m a NOC engineer). I thought it might be the HUD and disabled it, no resolution.