Oliver,
Thanks for getting in touch, and sorry to hear that you’re having this issue. I believe you are correct that this issue is happening because you’re loading jQuery-UI twice in your site. As I’m noticing on line 901 when I view the source of your Event Submission form, I’m seeing the following code:
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jqueryui/1.7.1/jquery-ui.min.js"></script>
I would look in your theme’s functions.php file or footer.php to see if that is being included somewhere. If you can find it, try removing that reference to see if it fixes the issue, otherwise I would reach out to your theme developer to see if they can help you remove it.
I hope that helps! Thanks! 🙂
-Casey-