Hi Craig!
I’m sorry you are experiencing this issue. Let me help you troubleshoot that.
Could you please enable WP_DEBUG and WP_DEBUG_LOG ? This will create a debug.log file inside the /wp-content directory.
After that, try making a few purchases of tickets, so that in case there is an error, it will be saved into the debug.log file. Please share that log file with us.
As this could also be related to a plugin or theme conflict, could you please go through the complete steps in this guide > https://theeventscalendar.com/knowledgebase/testing-for-conflicts/ and let me know what you can find in the process?
You might want to set up a staging/dev site for this, so you can make all of this tests without affecting the live site. Also, we always recommend you make a full backup of your site, both database and files so you can restore it in case anything goes wrong.
Let me know how that goes.
Thanks,
Victor