Might be maps, might be items, might be sprites within the current map, hundreds of possibilities.
Okay, to pinpoint the problem, you should try this:
1. Open up Data Packer, and remove mods, close Data Packer.
2. Go into your 'data' folder and cut or delete the folder 'db' elsewhere/recycle bin
3. Open Data Packer, 'Find' and 'Modify' mods, close Data Packer.
4. Go into Mabinogi and try logging in.
If it works, great! You've found that something in the 'db' folder is causing the problem. If you still crash, then repeat steps 1~4 except cut/delete the other folders in the 'data' folder one by one, trying to log in each time after cutting/deleting a folder. You can tell which folder is causing the problem because once you're able to play properly, then it means the folder you just cut/deleted was the problem.
Now, once you've found out which folder is the problem, go into the folder (if you deleted it, restore it via Recycle Bin) and then delete the files or folders one by one again. Basically do this for all sub-folders until you find out EXACTLY which file is causing the problem. Delete the file and you can play properly, but you should also tell Shaggy which one it is, so he can fix it up and in the next update it'll be fixed (probably).
P.S. If PT wants to add to guide, sure, you're gonna need alot of editing though since this was just quickly typed up.