That's not coming from Opus itself so it's likely to be a shell extension which has loaded itself into Opus.
Is FileVault something you use? I'd check with that first if it is, in case it isn't working properly in general and this is just one symptom, or in case there is an updated version which addresses the problem.
Many thanks Leo for your reply. I have no idea to what File Vault refers. I decided to download Directory Opus and reinstall (without uninstalling) and this seems to have stopped the error message. Most curious.