Client no longer receives the successful Thank You page, but rather it's a sort of shadow box

We just started receiving an issue with our forms. On a submission of a form, the client no longer receives the Successful Submission page, but rather it's a sort of shadow box. After checking everything on our side of the equation, it appears it may be on your end. Can you assist us with this? 

I was able to replicate the issue in a cloned form. Please try to update your Thank You Page using the Thank You Page Editor. Here's how:

In the Form Builder, click "Settings" and then go to "Thank You Page" section. After that, please do a minor change in the Thank You Page Editor like removing the "!" and then adding it back. After doing that, click the "Build" option in the top menu to save the changes.

Here's the result:

Hope that helps. Thank you.