NIK
I have had that same error, here is a possible solution.
If you have custom weapons you must ensure that they have a "vweap.x" file in the gun's folder in the gamecore. If the weapon doesn't have a vweap FPSC will crash as soon as you place an enemy.
1) Remove all the weapons from your level (including a player start weapon), and place an enemy and test. SHould run ok. Gradually add the weapons back in, until you find the one thats buggy. If you don't have a vweap.x for the enemy to hold, just copy a stock media weapon vweap in to the gun's folder.