The admin account should have received an email with the stack trace of that error. If you didn’t, please enable wordpress debugging, run the webhook again and check wp-content/debug.log for the stack trace.
To enable debugging, look for the following constant definitions in your wp-config.php file:
WP_DEBUG, WP_DEBUG_LOG, WP_DEBUG_DISPLAY
If you don’t have them, add the following:
define('WP_DEBUG', true); define('WP_DEBUG_LOG', true); define('WP_DEBUG_DISPLAY', false);
If you do have them, just make sure they have the values to match the lines above.