Good evening Matteo and welcome to the Events Calendar Support forum!
Thank you for reaching out to us.
We are sorry to hear about the 500 error you received.
I would love to help you with this topic.
It is quite possible that your customizations will require updating to work properly with the latest version of our plugins.
In fact, you might want to read the following: https://theeventscalendar.com/release-community-events-4-5-the-events-calendar-4-5-4-pro-and-community-tickets/
Additionally, I would highly recommend upgrading your PHP version to at least 5.4+. Older versions tend to produce undesired behaviours.
In fact, WordPress recommends PHP 7+.
Finally, a quick test that would be worth your while would be to revert to a default theme such as twenty fifteen to see if the issue persists.
Let me know how that goes.
Best regards,
Geoff B.