I have a similar problem, I'm using a Infinite Odyssey client, I can open, almost every dat file, but there's at least 2 I can't edit or save. Npcgrp.dat: I can edit and save the file, but if I do, npc glitches, and the NPC that look like PCs, those are the worst, the will appear as a floating head.
Armorgrp.dat: I can't save, and I don't need even to do a thing, if I open it and try to save right away, File edit doesn't do any thing, and there's not even a error message. Any suggestion what that might be? I'm using the File Edit for Infinite Odyssey, and I even have edited some files, those 2 are the only ones I can't edit.