Bootstrap "modal" doesn't work in theme

Hello together!

This is a very tricky one and we’ve already tried ad dozen of possible solutions –?but without any success ??

The setting:
appointmed.com (which is not a WP installation in a whole) uses a custom, minified .js that already contains jQuery and all needed functions for the theme. WordPress is installed in the appointmed.com/blog sub-directory. We adapted the original theme to fit WP requirements.
We unregistered the native WP jQuery in the functions.php and registered and enqueued the file from the original theme directory (appointmed.com/assets/js/…). wp_head() and wp_footer() are implemented correctly in the theme.

The problem:
The script seems to work partially as the sticky header and the floating box in the sidebar are working correctly. BUT if you click on the big blue button in the top right corner, a modal window should become visible over the whole canvas (you can see it working e.g. on appointme.com/team.html or any other page except the /blog pages.

We’re running out of ideas why this modal window doesn’t show up and hope you have some ideas what we could try. I bet it’s just a small tweak, we’ve overseen somewhere…

Thanks a lot,
Mario