I'm working on a town expansion mod at the moment and have come across these errors when testing the mod ingame:

"Failed to load spell resist_fire_75"
"Failed to load spell resist_disease_75"

These appear after I've added new NPCs (Dunmer for the fire one, I assume, not sure which kind uses the resist disease). The errors only appear once - i.e. if I save my testing game, then reload, they go away. The resist fire one seems to have popped up for every Dunmer I've created (I think).

What could be causing this problem?

Those two spells are not from the original master files. They are added by a mod. It appears that they are referenced from somewhere (like a script, race definition, birth sign definition, etc ...) but are not defined in any loaded file. That usually requires that they did exist, and were referenced, but were later removed.

You will need to find the offending mod amongst those you have loaded. Note that the original master file contains ability spells with similar, but different, ID's.

That's weird. I test my mods on a different Morrowind install, so the only ones I had loaded were the one I am currently working on, and an earlier one I recently made amends to. Both are made using only Morrowind.esm, though I have both expansion packs. (I deactivated the earlier mod and cleaned my current one with Tesame, cleaned my savegame with Wrye Mash). Where can these spells have come from if they're not in the original master files? :S

So try a game without either mod, and then try a game with each individually. Start new games to be sure any saves are clean.

Ok, I did that and didn't encounter any errors. Then I added a new Dunmer NPC into my current mod and loaded the new savegame I tested said mod with, no errors there either. I suppose the older savegame was corrupt somehow.

By the way, I found "resist fire_75" in the CS, so it must be in the original master file

(Oh, just realised I may have noted the error down wrong with the additional underscore. If the error pops up again I'll double check that.)