quicktime 7 Pro... getting the registration working even though the server is down?

I've done everything properly.

Not exactly. To activate the Pro license, do NOT click the ‘Buy QuickTime Pro’-button. Instead, select one of the other text fields (with the mouse or the Tab key). A small ‘Pro’-sign should be added to the Q-logo on the left, as confirmation, then close the Registration window with the Close-button.

The license will be accepted if both the name and registration code are exactly as in registration email, without need to contact any server.

The Buy-button would take the user to the site to buy a new registration code, but that site has been gone for many years, as new Pro registrations aren’t sold anymore.

QuickTime 7 Pro registration worked at least a few weeks ago (in Mojave). It registered instantly (AFAIR you must hit the Return key to apply the registration code).

If that fails, MPEG Streamclip is a good alternative to QT 7 Player. It still works in Mojave but you may have to Get Info > Low Resolution to get rid of display artifacts.