You've definitely checked all the EXEs and DLLs, and seen "The digital signature is OK" for each one?
(Note that just seeing something in the Properties dialog's signature list isn't enough; you have to click the Details button to actually test the signature, as shown here.)
Another thing to check is that you are using the right licence/certificate, and not e.g. an old evaluation certificate found from an old email by mistake. You can check that via the Help menu, then Licence Manager. Where it says "Certificate Details" it should say "Type: Normal" rather than "Type: Evaluation" or similar.
If it's neither of those things, do you see this message the moment Opus starts or later on? Do any Opus windows open before you see it?