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.