A-ha. I found out that's the reason why I keep getting this error.
But I still seem to have issues with recovering MP. I still can't seem to recover MP, but at least I found out about what keeps causing this error.
Apologies for double posting (and necro posting), but I decided to try out Joewoof's common event method.. At first, it didn't work, but when I tweaked it a little bit, it finally worked!
Thank you so much Joe (you too Boonty and udime)! Now I feel very satisfied.
Salvage plugin. Craft something you decide you no longer need, or find an item you have no interest in, salvage it for scrap and resources. As you salvage items, gain EXP and level up. The higher your level, the better odds you have at succeeding, and the more material you'll actually acquire.