[Plugin: All-in-One Event Calendar] Filter Not Working

We’re running AI1EC version 1.5 – https://www.ads-software.com/extend/plugins/all-in-one-event-calendar/ – and it seems to work well in all respects, except that on the main calendar page – https://www.councilofsocialservices.org.nz/calendar/ – the categories filter is broken.

Clicking the categories button correctly presents a drop-down list of categories, but none of these categories are clickable.

This feature had been working earlier, but we’re not sure when it stopped. It could’ve been when we upgraded to version 1.5 recently, or it might have first occurred while we were still running the previous version.

The complete list of other plugins we’re running is:
– Akismet 2.5.5
– Contact Form 7 3.1.2
– Google Anlaytics for WordPress 4.2.4
– Google XML Sitemaps 3.2.6
– Really SImple CAPTCHA 1.5
– The OpenHook Customizations Manager 3.3.1
– WPtouch 1.9.39 (currently deactivated)