Unfortunately, Apple is now blocking sandboxed apps to change the default handler for a particular URL scheme. This is why Screens is not able to set Screen Sharing back as the default handler. This change affects a whole bunch of apps that use to rely on this functionality.
If you’ve purchased Screens directly through us, make sure you update to version 3.5.3, which solves a related bug on our end that may prevent setting back the default handler to Screen Sharing.
If you’ve purchased Screens on the Mac App Store, you can download this small utility to fix the issue:
A new version has been submitted to Apple, which will now remove the feature. We’re always sad to remove functionalities from our apps but sandboxing gets more restrictive every OS X release.
Update: Screens for Mac 3.5.3 is now available on the Mac App Store but you may need to run the Screens Default VNC Reset utility to fix the issue still.