Hello IJWAS,
It might be the issue of plugin conflict or cache issue version compatibility issue. Please make sure your theme, plugin, and WordPress version are updated as per our version compatibility page. I would suggest you must troubleshoot a few of the common issues. I checked your given page URL it seems that the register button is working but the form is not displaying the modal. For a quick fix, you may add this custom CSS code under Theme Option —>CSS.
.lity-content .mec-events-meta-group {
display: block;
}
The purpose of providing custom CSS is to show you how to add CSS code to your site. Writing custom CSS is outside the scope of our theme support. If you need more customization, you need to learn CSS and learn how to use the browser’s element inspector.In case if you have no idea about coding you can subscribe to One where customization questions are answered.
Hope it helps.
Thanks