Hey Jonathan,
Thank you for reaching out to us! I’m very sorry about this issue you are experiencing, let me help you to solve it!
I’ve seen similar issues in the past and in those cases the problem was associated with a theme conflict.
Can you please temporarily deactivate all of your plugins (except Modern Tribe plugins) and change your theme to another one like Twenty Seventeen and let us know if the issue persists after that? You will find further information about our conflict test here.
Also, please enable WP_DEBUG and WP_DEBUG_LOG and share with us any messages you see while creating new events. All WP_DEBUG messages will be stored in a debug.log file in /wp-content/debug.log.
Thanks!
Patricia