I'm have the same problem as described in the original post by YPhil. I'm using the Pro version. Windows 10 (version 1803); anti-virus is Windows Defender with Malwarebytes. This is a significant problem, it needs a resolution. I have finally uninstalled the Ccleaner and reverted to my original Pro purchase (revision 5.24). How do I install 5.44 Pro so it works and recognizes my registration code? Also, in trying to install 5.44, which was a Ccleaner pop-up recommendation I some how install Avast which I didn't want. This is really irritating and YES I always watch and try to uncheck the installation of extra software. I saw nothing saying Avast would be installed.