In this issue, someone mentions to follow the following solution: Online Multiplayer - Error when queue_free() player - #11 by p0nder.
“It seems there’s about a 1 or 2 frame delay with MultiplayerSynchronizers that causes this error when its parent scene is removed. So turning off the public_visibility right before removing the parent scene resolved this for me. However, I still got the error if I queue_free()'d the MultiplayerSynchronizer so it seems that turning off public_visibility is the only way.”
Maybe its the same for adding scenes