Front-End Locks Up/Times Out When Choosing Provider

This issue just appeared out of nowhere. We're unable to accept appointments because when the user selects a provider, the website locks up and finally times out with a 500 Internal Server error. This only happens when a provider is selected no matter which provider. A service can be chosen just fine. I've tried disabling all other plugins to ensure there wasn't a conflict to no avail. Here's the URL: http://www.carcareclinicjetlube.com/book-your-appointment. Thanks for any insight. Jim