No Input and Errors after creating a session


I am creating a 2,5 D Multiplayer-game. But I have some problems. Today I created a basic Session-System. But when I join, I have no input, and I get the "Blueprint Runtime Error: “Accessed None trying to read property…”-Error. I add some pictures, so you know what I am talking about. The creation of the sessions works pretty well, but after I joined, there is no input, and when I stop testing, i get these errors. I already tried to set the “DungeonCharacter” at EventBeginPlay, but that has no effect.