Sandbox in multiplayer is not possible.

[y.petremann]

When I start a game in singleplayer in sandbox, then save and load it in multiplayer, I get a black screen., I could potentially play, but in the middle of nowhere.

[kovarex]

Hello, thank you for the report, we made the needed fixes, and the sandbox in multiplayer now works (at least it works for us^^).

So it is fixed for 0.11.6