One of the symptoms of this error is that the X (close) button on your lightbox doesn't work and the lightbox isn't centered on your screen.
In order to have a working lightbox, try to check and verify your Facebook Settings if the you've added the correct Facebook API id and Facebook API secret.
If Facebook Application didn't verify your app, update your settings to match with the error message:
"The app you are trying to connect to is set for another Domain. You must have a unique Facebook app for EVERY domain that you wish to run Hybrid Connect on as specified by Facebook's security policy"
The app with the the id and secret that you've entered is already registered to the domain : THEDOMAIN
You can check and test this error by changing your connector type to "Form" and check your lightbox again, if it's working, then the problem is your Facebook Settings.
If this isn't fixed, please contact support for further assistance.