Hello!
Welcome back and sorry to hear about the troubles. I see you checked ‘Yes’ to both reverting back to the default WP theme and deactivating all plugins other than ours as recommended in the Testing for conflicts guide, but can you please confirm this? It is a required step and will help narrow down the source of conflict.
Can you add the following lines of code to your wp-config.php file and visit the Attendee list (same screen as the one in the screenshot you shared with me)? Hopefully this can help point us in the right direction.
define('WP_DEBUG', true);
define( 'WP_DEBUG_LOG', true );
Thanks and have a great upcoming weekend. Cheers!