Hello Jacopo,
Isn't there also a message suggesting to turn off the Gatekeeper?
It seems that your Mac OS is moving PhotoScanPro application bundle to some temporary folder. So you need either to disable Gatekeeper, the following terminal command should work, but maybe there are some other ways:
sudo spctl --master-disable
Alternatively you can activate PhotoScan from the command line:
PhotoScanPro.app/Contents/MacOS/PhotoScanPro --activate 11111-22222-33333-44444-55555
Where the last arguments your license key.