JS not working properly, wheel keeps spinning

Hello, it seems I have a theme related issues. When I try to add a menu item, the “save wheel” keeps spinning forever. The same goes when I tried to send an email through Contact Form 7, I had to disable JS to fix it up quickly. Now I am using a plugin that requires JS to work and I have no idea what is wrong with the theme.

I am not the author of the theme, it is the work of our former employee. He himself is not able to find the solution, so that is why I am here. Could you please tell me how to find out what is causing the problem? I am completely sure that it is theme related, as I’ve already tried to switch to the Twenty Twelve and it worked. It’s also not plugin related.