Mcreator155 is not compatible with a minecraft server, That is normal ??
MCreator is compatible with the Multiplayer mode but there are some bugy Mods parts which cause this Problem
After I am back home here is a List of stuff that has Multiplayer Problems:
(From the Changelog, but /todo list:
Problems with SMP: basic GUI, key bind
And there is also a Problem with the Custom Rendered Blocks