Pro Sites Stripe Gateway Error: Invalid timestamp

I'm getting an error with the Stripe gateway. It seems to have begun with a recent Pro Sites update because I've never seen it before. This is the error that shows on the checkout page:
Invalid timestamp: must be an integer Unix timestamp in the future.

I temporarily fixed it by re-saving the Pro Sites settings and Levels, but after one successful transaction it has appeared again. I'm using the latest version of the Stripe API.

Any ideas on fixing this? I'm unable to signup any new customers.