Its been a while but now i have some time again.
So your answers throw up more questions.
1) My whole movement logic is some calculation based on the values of the TileEntitys, which are set once and dont change after.
So my first thought was that server and client should probably do the same calcualtion with the same result.
2) Descriptor Packet. Is there some packet method to send the data, like those in the TileEntity class?
3) IAdditionalSpawnData. What kind of thing is this?
Google has 3 results that contains this word. One is this post. I cant find a thing called like this in my whole workspace.