UPDATE: This resolved itself. I first uninstalled/reinstalled Cubase and Dorico, rebooting after each step, and the error was still there. But then after a short wait, during which I opened the Security settings in Windows, the error just stopped happening. So I don’t know.
On Win11, I just installed the latest Dorico 6 Pro, and it won’t launch. Smart App Control is complaining about ‘Qt6QmlMeta.dll’ and ‘Qt6QmlModels.dll’ not being signed. I uninstalled and reinstalled. Any ideas? Thank you! 
Those 2 DLLs should be inside the Dorico program folder under C:/Program Files/Steinberg/Dorico6. Are those 2 files there? If so, check also on the file permissions of them and that you as a normal user have read and execution permissions.
Thanks, Ulf! Those modules are there, yes, and regular users have read & execute perms. However, the error message seemed to be about the modules not being signed, so I guessed Windows was choosing not to execute them rather than not being allowed to. Anyway, see my updated OP for deets of how this has resolved itself.
Well, if it works , it works, so let’s not bother any more.
But do come back should trouble come back again.