If any "unofficial" versions (or related techniques) were used on the same machine in the past, make sure nothing is left of them.
If in doubt, do a backup of your config (using Settings -> Backup & Restore, storing the backup somewhere safe), then uninstall Opus (which wipes the config, which is why you want the backup). After uninstalling Opus & rebooting, make sure nothing else is left in the C:\Program Files\GPSoftware directory.
Check the digital signature on your installer, and if it's OK run it to install Opus again. You can restore your config using Settings -> Backup & Restore, along with the backup created earlier.
When installing your certificate, make sure it is the current version of your cert, which you can get from either My Account or Lost Registration on the main GPSoft site.