I am also interrested in a solution here.
This issue also happends when with the following steps :
- Open the project in the editor, open the wanted map
- Launch a server through command line using UE4Editor.exe [ProjectName].uproject [MapName]?listen -game
- Launch PIE from editor (I tried VR mode and New Editor Window, got the bug in both cases)
- Connect to the server using open 127.0.0.1
- It works fine up to this point
- Close PIE, kill server with Alt+F4
- Try saving the map in the editor. It crashes with the same error message.