Hi Benjamin.
Although this isn’t the issue you requested assistance with, I noticed console errors at your site:
(If needed, you may reference our KB article Using Google Chrome Developer Tools.)
You should deactivate the other plugin(s) or theme that is including Google Maps API on The Events Calendar pages. Additionally, you should input your Google Maps API key at wp-admin > Events > Settings > APIs > Google Maps API
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To your question, I didn’t notice any JS errors when clicking the “+” button, but I’d still suggest running this past your theme developer because the plus/minus buttons aren’t part of Event Tickets Plus’ default behavior for WooCommerce tickets. You can see such a demo at http://wpshindig.com/event/events-tickets-demo/
I hope all this information helps you with your site.
Please let me know if you have any follow-up questions on this topic.