This means a battler has a non-numeric value for their HP. The default code has a safety check in place to prevent this error (non-numeric values become zero), but I'm guessing you're using Yanfly's Damage Core plugin, which removes that check.
The cause is likely an error in a skill formula: something doing non-numeric (e.g. undefined) damage. It could also be worth checking any scripted damages, i.e. anywhere you've used gainHp.
I always told my DA fans how much I hate slot machines. They're fine in games as a risk-and-reward system. But when you're spending REAL MONEY in a Vegas casino to try and hit the jackpot (which very, very few people will), it can really hurt your budget. Gambling is a bad habit, and I don't like wasting my money on a slim chance. Go to Vegas for the experience, not the jackpot.
Still no scam calls all day... also I got a quick gig designing a mascot drawing for a small business, so I might actually make some green of my own after all. That would be a perfect job for me, since I love art and digital design.