During the fight against the Bard, Jimmy uses an ability that prevents me from using mana. The mana bar turns into yellow and black caution tape. I used a cure potion to remove debuffs during the battle, but it nothing to make my mana usable again. Now I've finished the battle, but my mana is still caution tape. Is this a bug, or am I missing something?

Caution doesn't mean you can't use it, it means you have too much and are about to crap yourself.

 

If you're actually unable to use it, it sounds like it skipped updating the UI. Because when you do crap yourself, it drains all your mana. Try using a mana potion. See what happens.

Thanks, I'll give that a try.