Hope this helps someone, I luckily only lost half a day on this.
My game was working completely fine in 5.5. Upgraded to 5.6, and suddenly was crashing on traveling from my Lobby Map to my Match Map.
After lots of digging and trying to figure out what the error was pointing to:
[2025.07.08-20.42.18:048][ 54]LogOutputDevice: Error: Old World /Game/AllBlueprints/Maps/UEDPIE_0_NewLobbyMap.NewLobbyMap not cleaned up by GC! Garbage object ScrubbedSound /Game/AllBlueprints/Maps/UEDPIE_0_NewLobbyMap.NewLobbyMap:PersistentLevel.ScrubbedSound_Root_0 was previously being referenced by NULL:
[2025.07.08-20.42.18:048][ 54]LogOutputDevice: Error: (refcounted<1>) (Garbage) ScrubbedSound /Game/AllBlueprints/Maps/UEDPIE_0_NewLobbyMap.NewLobbyMap:PersistentLevel.ScrubbedSound_Root_0
I finally found out that this is caused by Audio playing INSIDE an Animation in a Widget.
Once I removed the Audio from being embedded in the Animation, and just put it in the Blueprint Event Graph as just a normal “Play Sound 2D”, I was able to map travel again just fine.
It looks like Levels cannot close properly if sounds are actively playing inside an animation which causes the editor to crash out.
Hope this helps someone! Good luck all!