Built from commit 98d02ac
When placing a tank or a pipe in the world, the following exception occurs on the server:

Tanks: http://paste.cisien.com/f/b6c5644c.txt
Pipes: http://paste.cisien.com/f/a5c652b2.txt
Pump: http://paste.cisien.com/f/a330e787.txt
Path Marker: http://paste.cisien.com/f/83648e3b.txt

I'm not going to continue listing each item that crashes for this issue, just assume that all buildcraft mods/blocks are affected.

This is an extension of #1437 I think

Indeed. I'm going to need some help here. I can't seem to be able to connect to a server from my Eclipse build environment. Everytime I try to connect to it, I get:

[23:53:40] [Server thread/INFO] [net.minecraft.server.network.NetHandlerLoginServer]: GameProfile{id='null', name='Player524'} (/127.0.0.1:54346) lost connection: Disconnected

Anyone knows what that is? Makes it difficult to run a debug session...

It looks like you are connecting with a fake username (given the lack of id), make sure the test server you are using has online-mode set to false in the server.properties. Or try using an actual minecraft account.

or it may be offline-mode set to true

I'm using a development environment, so it's indeed a fake id. That's probably the issue of online mode set to false, thanks! Now to investigation mode again.