@mikaelK the “Getting started” in the documentation has been out of date since 0.3.
I posted the same question to @timjohansson [here]( DOTS Multiplayer discussion page-5#post-6554188)
When did the error start occurring, when you added the cube? Try following the “get started” and hitting “play” after you make the client/server connection and see if the error still appears.
This might have something to do with it: [Netcode] prediction jitter
also reported here: Issues with netcode 0.5