Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
Guide on Entering A QuickTime Pro Registration key in Mac OS X
For Mac OS X v10.6 Snow Leopard
• To install the product, insert the Mac OS X 10.6 Snow Leopard DVD and open the Optional Installs folder.
• Double click on ‘Optional Install.mpkg’
• Move on to QuickTime 7 and click on ‘Continue’
Once done, Mac will install QuickTime 7.
• If you do not have the QuickTime 7 Pro registration code on your computer, you have to manually enter it. For it, open QuickTime 7 Pro
• Click on QuickTime Player 7 and move on to ‘Registration’
• QuickTime Registration window opens up. Enter the code and press ‘Buy QuickTime Pro’ and you are done.
For Mac OS X v 10.5.8 Leopard or before:
• Go to the Apple menu and click on ‘System Preferences’
• Move on to ‘View’ then QuickTime
• The registration window will pop up. Fill the information and you are done
Keep in mind that some older Mac Os X versions might have Registration button in the Plug-ins tab or Edit Registration option. Click these buttons to launch the registration window.