WinCC Flexible 2008 "No valid license key found"

After upgrading the HMI with OS XP to the new HMI with OS windows 10 the WinCC Flexible 2008 SP5 could not be opened due to a License issue. So when I open the WinCC flexible get the message "No valid license key found". The app will be closed" (image 01). The system displays a message at short intervals informing "unlicensed version" and then closes (image 02). After that, I open the Automation License Manager and get a pop-up window with the alert "one or more unused upgrade license keys were found on your local computer. You can perform an upgrade with the menu command "License key > Upgrade" (image 03). Once I perform the upgrade warning message pops up "No matching upgrade license keys were found" (image 04).

Why is such a warning message popping up even though the keys are on the local disk?