Gravity Form in pop-up won't submit

If you click on the button at the bottom of the page to request more information, a Gravity Form comes up in a pop-up, and it's been working just fine. Today client contacted me that it has stopped working. When you try to submit the form now it just spins. It does seem to actually submit the information actually -- but it never looks like it has done. You don't get the on-screen confirmation message.

I tried opening the form on just a normal page, and the form works just fine. It is just in the pop-up that it won't work.

Everything is at the latest version.