I found the source of my problems :
LAN vs. WAN
Our platform uses preferably LAN mode, but if not available it will use WAN.
In development, LAN is always used since the database is LAN reachable (if not even on the same computer!).
I have put my implementation code in a server assembly, but when in LAN, the server (clientside) cannot find the implementation. Now I moved my code in a "shared" assembly thats delivered both on the application AND server: its get called and it works!
I forgot about that DevForce part of LAN/WAN and corresponding EntityServer...
thanks kimj for your support