BuildCraft version: 7.99.16
Forge version: 2673
Link to crash report or log: https://gist.github.com/D3nnis3n/02972a418660d63f4337b37323cab43b
Singleplayer or multiplayer: Multiplayer
Steps to reproduce: Start server, login near the mentioned block, server crashes
Additional information: This happens for a hell lot of blocks, so removing was only possible through removal of buildcraft.

Hmmm, that means that the noppes.npcs.api. package is still present, and probably contained by a mod.

java.lang.ClassCastException: mcmultipart.util.MCMPWorldWrapper cannot be cast to net.minecraft.world.WorldServer

at noppes.npcs.api.wrapper.EntityWrapper.<init>(EntityWrapper.java:120)

I don't think this is buildcraft itself crashing - do you know what mod contains the noppes.npcs package? (The mod name will probably be similar to the package name)

Custom NPCs - which we do not use at all. It's just installed.

Ok, can you try removing it?

(EDIT: Just to check to see if it really is the cause, we should report this to them if it is)

I can't reproduce the issue. The pipes at that place have been placed for three hours. Just some time they crashed. I'll remove it - but i can't know if it worked, as this issue does happen out of nothing.

//EDIT:
The pipes were not even in use.

It's now removed.

Another info:
The server crashed when noone was on the server at this time, therefore noone was near the blocks.

After that it crashed whenever someone got in the near, after server restart.

Does it generate exactly the same crash report every time?

Yes, but it's another block everytime. The Block Coordinates do change. We removed around 12 fluid pipes, then it was okay.