Hi FabioZ,
In order to resolve the issue, please do the following:
• Open Finder
• In the menu bar go to Go -> Go to Folder...
• Type in '/etc/' (without quotes)
• Find hosts file, right-click on it -> Open with... -> TextEdit
• Remove everything which contains 'parallels.com'
• Save the file
• Restart your Mac.
• Start Parallels Desktop and register the program.
Thanks,
Abdul