I upgraded to Ccleaner Professional this evening. After restarting my PC, I can now start Ccleaner but it shuts down a few seconds later. It does this repeatedly, and after another restart. Please advise.
Closing a programme shortly after it is started sounds like something that an antivirus might do.
Are you using one other than Windows Security/Defender?
No.
Have you actually managed to open CCeaner and enter your Licence/Activation key?
Something may have glitched during the upgrade so I’d try doing a reinstall of CCleaner.
Download an installer from here: Download CCleaner builds | CCleaner Portable, Slim, and more
Once downloaded double-click the downloaded installer to reinstall CCleaner.
On the last page of the install where it asks if you want to open CCleaner leave it ticked so that it will open aitomatically. (You don’t need to view the release notes, you can intick that one).
When CCleaner opens then if you hadn’t managed to enter your key before you will need to go to Options>Licence Key to input your key and activate it as Pro.