Here's it's own thread to track this issue.
Using the provided Damage.lua script, I am passing 2 spell_data params, both INT, one is damageType 0 (slashing) the other is damage amount 9, for the Assassin skill "Quick Strike" (I am not currently using the DOT component). After casting it once, I see the right damage hit the mob - but after casting it a second time, I usually crash the world. See here two casts - both reporting something about a nil value - what is this?
My Damage.lua (from SVN):

--[[
This is the EQ2Emu generic spell damage script written in LUA.
These functions are shared, so don't save any character data in them.
If you have any questions be sure to read the readme.txt file located in this directory
--]]
--main process function
function cast(Caster, Target, DamageType, Damage)
-- syntax for SpellDamage is Target, DamageType, and Damage
SpellDamage(Target, DamageType, Damage)
end
function remove(Caster, Target)

Info from servers Application log: I hadn't noticed ntdll.dll being hit before.
Edit OP: Renamed because this has nothing to do with spells only. I see more world crashes changing LUA scripts, and using /reload luasystem and /reload spawnscripts then /repop. Seems to happen often doing that.

I cant reproduce this. Even setting it up like you have it wont crash my server. Could you dump your entire database to a file and upload it somewhere for me? Maybe it is something not obvious. :/

Posted on DB SVN under Trunk/TestData. I doubt there is sensitive data in there but delete it when you're done please.
I had been trying to build a Heal script too, but of course having no luck with that. I am simply worn out, man. lol... thank god I have to work tomorrow.

Rev 122 Crash just now:
This crash was brought to you by /reload spawnscripts, randomly. Since I had done it 12 times, with repops and /reload luasystem commands too.
Scat and I were the only two on, and he was AFK at the time.

Went ahead and renamed this post to cover the general crashings I am experiencing. The good news is, if >I< am not logged in, the server does not crash. So it is definitely something I am doing.

Just had another world restart. I had /reload luasystem and /reload spawnscripts after fixing a quest, and /exit because I had already done it and wanted to clear the data so I could do it again.
The zone was still running (could still see sapswills being murdered endlessly) and when I clicked Play, I saw my acct key and crashed. It still could be the /reloads... since I logged out and back in quickly.

LE, maybe good news. I removed the wandering, dying invaders last night from the world so they were not spam-dying. I then left myself standing in-game all night while players were in and out. There was not one crash.
Very first thing I did this morning was /reload spells, and 20 seconds later, the world crashed. Hope this helps narrow it down. You can export my DB if you need it.

Here is the latest update using the dev 409 server. While testing the queuing fixes, I was watching the console as Scatman was queue-casting. When it appeared he was done, I sent a /reload spells and immediately crashed EQ2World.exe.
I am not sure if it was trying to reload while a spell was being cast, or cooling down, or whatever. Just more info.

New (old) confirmation... this is NOT limited to /reload spells. It is now happening while I am /reload luasystem the /reload spawnscripts (not sure if both are necessary for reading newly created scripts).
It crashed just before I got to the /repop, so it's in /reload, fer sure.