First of all, we would recommend you to update theme to the latest version (6.1) because you are using out of date version 4.7.3 and it can be the reason of your issue.
Also, it you use child theme and custom php code there it also could be the reason. And finally, it could be just compatibility issue with third-party plugin.
So,
1) Provide us with WP Dashboard and FTP access to your site to update theme and check if it helps you. Don’t forget to make backup of the files and DB before replying.
2) Describe in more detail what errors do you get while enabling modern event plugin, provide screenshots, describe your actions step by step to reproduce the issue and help you.
