Hi Marco,
I guess a corruption of the config_4.xml file. This file holds the license key.
A newly generated free license is not compatible with version 6.x.
If you still have the old 6.x version license key, you can use it. If not, try the following:
Exit the Viewer.
Make a backup of your "C:\Users\[Your_username]\AppData\Roaming\Remote Utilities Files\" folder, including subfolders.
In that folder, under "backups" folder, you will find a xxx-xx-xx_config_4.zip file. Extract the config_4.xml file and copy it in "C:\Users\[Your_username]\AppData\Roaming\Remote Utilities Files\" folder (overwiting the current config_4.xml file)