As per the MobArena integration page I added "example: arena" to my plugins/Magic/config.yml then I added the suggested mage class but I received 10.06 22:59:08 [Server] INFO Failed to parse armor for class mage: magic:wizardhelmet as an error. That also happened for each of the items when I added them to an already working class. Let me know what you think.

+1 to this. After updating my server to 1.16 and updating my Magic and MobArena plugins, my Mage class no longer works despite making no changes to the configs.
Magic version: 7.9.10
Mob Arena version: 0.104.2

Do you still see this message in your logs?

[Magic] Integrated with MobArena, use "magic:" in arena configs for Magic items, magic mobs can be used in monster configurations

I'm not sure what could be going on there since it's still working for me :(

Sorry, any updates here? It's still been working on my dev server fine.