Thank you for your message.
There might be two possible causes:
1. The formatting of the key that you are copying is broken. This happens often when you copy the key from Outlook (this email client may break the formatting of our license key if you copy directly from message body).
2. The Viewer configuration file is corrupted. To fix this close Viewer, then navigate to %appdata% folder , then "Remote Utilities files" folder and locate config_4.xml file. You need to delete this file then restart the Viewer and apply your license key.
Hope that helps.