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.
Letting people know I'm debating leaving RM, and thus this forum. I'll continue to keep in touch with any friends I've made here, but between Unity being better suited to my needs and people on here getting increasingly horrible, I think switching my games to Unity might be better for my mental health.