Hey Heinrich,
Thanks for your patience with us over the holiday break!
It seems that you are using WooCommerce Tickets on your site. This plugin can no longer be used, and you need to change over to the new Event Tickets plugin.
Change to the proper plugins and see if your issues persist.
So, first, install the free core plugin Event Tickets → http://wordpress.org/plugins/event-tickets
Then, go to http://theeventscalendar.com/my-account/downloads and download and install Event Tickets Plus.
Activate both plugins.
Then DELETE “WooCommerce Tickets” from your site.
Then see if issues persist.
Let us know what you find!
And please be sure to read this article, which covers the plugin change in more depth → https://theeventscalendar.com/knowledgebase/moving-to-event-tickets-plus/
Cheers!
George