In Heroes of Might and Magic IV (aka Heroes of Might and Magic 4, HOMM 4, HOMM IV) this error occurs when the name of the executable used to launch the program differs:
multiplayer_msg_general_failure |
The content of the executable does not matter, it is the name. So both must use for example h4tour351.exe, even though in Equilibris 3.51 it has the same content as h4mod.exe.