There are a billion things that can cause this. Fucked up custom map, fucked up custom NPC, fucked up custom weapon, etc. There's no straightforward way to find out what exactly is causing it. And it is unrelated to memory itself.
My advice: make a backup of your system first of all. Then, one by one, replace the dats on your system for the retail ones.
First change:
npcgrp, npcname
run l2.exe and log in
if it keeps crashing, go etcitemgrp, weapongrp, armorgrp, itemname
if it keeps crashing, replace lineageeffect / lineageskilleffect, try again
if it keeps crashing, replace 22_22.unr with the retail one in the maps folder
etc etc etc
that way you identify where the problem comes from. If it gets fixed when replacing npcgrp/npcname? then it's a custom npc fucking you up.
Another, faster way, delete your whole system and try the retail system with modified l2.ini to be able to log in. If it still crashes, then it is an issue outside of the system folder. Replace the external folders one by one (animations, systextures...)