[CustomNPCs 1.5.2] - Hexxit - Server bug?

[problempants0000]

So, we have a 5 player server running the Hexxit modpack, with the sole addition of one extra mod, CustomNPCs.

Something about CustomNPCs doesn't play nice with the server. Only three players are playing it, all of them have CustomNPCs installed in their own game files, and we have CustomNPCs installed in our server. We can all hold the tools and use the custom recipes added by the mod or by players using the NPC spawner tool.

However, I'm the only one who can actually use them. If either other player uses them, they immediately crash and are booted off the server. I can make NPCs, the NPCs can detect them and attack them, all that, but they cannot see them, nor can they interact with them in any way.

What could be causing this?

I will add, as far as I know, I'm the only one to have run the modpack in single player as well as in multiplayer, I believe the other two have only run it in multiplayer for the server. Could this possibly be part of the problem?

Any help would be appreciated.

[TileEntity]

Add such a crash-report here