Here's the crash error shown in MMC Console: http://pastebin.com/cHpieet1

The crash report is here: https://dl.dropboxusercontent.com/u/76791645/Crash%20Reports/ShiftC.log

While on a server, I went to use the 'C' function to craft something, but held shift as well. This caused my client to crash, I have replicated it a couple of times.

Cheers for that! I know this won't be high on Pahimar's fix list, (if indeed at all) as I know he's still looking to fix the crafting one that's been bugging him for a couple of months.

Only trying to help anyway. I'll just avoid sneaking for now!

That happens because EE3 tries to open a portable transmutation GUI when you hit shift while holding C. That GUI is, however, not completely defined in EE3, and is constructed with just the argument null - resulting in a NullPointerException. A quick fix would be commenting out this line, resulting in the Minium Stone not doing anything on Shift + C:

openPortableTransmutationGUI(thePlayer, itemStack);

so that the code will look like this:

if (!thePlayer.isSneaking()) {
      openPortableCraftingGUI(thePlayer, itemStack);
}
else {
     //openPortableTransmutationGUI(thePlayer, itemStack);
}

As soon as Pahimar finishes the portable transmutation GUI, the comment can be removed again.
But whether he wants to do that or not is up to Pahimar.

Edit: Just tested it in my Eclipse workspace and shift + C doesn't crash me anymore.