Is the site in question running the current version of WordPress? If so, you can troubleshoot this by following these steps:
Install the Health Check plugin, then go to WP → Dashboard → Heath Check → Troubleshooting. There you’ll Enable Troubleshooting Mode, then once that’s activated you’ll reactivate Event Espresso and the Attendee Mover add-on only via the Health Check’s Troubleshooting item in the WP admin bar. At that point, can you check to see if the issue persists?