Hey Alex,
Thank you for reaching out to us!
To get started, could you please make sure that all of your plugins are up-to-date? Also, please share your system info with us.
REgarding the 500 Internal Server Error, this is a very general HTTP status code that means something has gone wrong on your website’s server, but it could not be more specific on what the exact problem is. This status code is usually associated with a PHP Fatal Error, so to identify the cause of this issue, I would recommend you to temporarily enable WP_DEBUG and WP_DEBUG_LOG and share with us any messages you see while trying to RSVP to one of your events. All WP_DEBUG messages will be stored in a debug.log file in /wp-content/debug.log.
Thanks,
Patricia