Hi,
Thanks for reaching out to us! I’d be happy to help you 🙂
I noticed your Event Tickets plugin is not up-to-date, could you update it to version 4.8.1? The same with The Events Calendar: could you update it to its latest version, 4.6.23?
Also, please make sure all your other plugins and your themes are up-tu-date.
In case the problem persists, I would recommend you to test for conflicts. In order to do so, you should deactivate all your other plugins to see if that makes a difference. Then, you may switch to the default theme (Twenty Seventeen).
Also, 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 ticket purchase, so that in case there is an error, it will be saved into the debug.log file. Please share that log file with us so we can spot the problem from there.
Cheers,
Jeremy