At the moment a lot of functions and classes from the minecraft client packages are called all over the place. This should be fixed by either eliminating variables or functions with @SideOnly(Side.CLIENT) or making sure that the functions aren't executed in the first place (like checking if we are on the client side or by providing dedicated methods that are only called at client or at server side).

For now this should be fixed, except for some minor bugs here and there.
In the long run however, we might have to consider to rewrite some parts with the purpose of having some cleaner code.