Fixed! Turns out gson was trying to serialize the entire references to the bukkit config file, as well as the runnable listener (which is being used to detect level ups), which caused some nasty circular reference issues. (the json files ended up being almost 8mb before I fixed it!)

Edit: I forgot to click post, but since writing the original part above I managed to get a working mana display above the hotbar, and the wands now use mana. In doing so I broke player loading, and I also need to figure out a better way to store mana stats, since if I make the entire mana class transient then the player’s max mana will also be transient.

All i had to do to fix was add the transient keyword to the fields that were causing issues.