Rick Altman wrote:

All at once, and for reasons unknown, I have lost the ability to
invoke Outlook from other applications. For instance:
....snip...
I have been doing this
successfully for years upon years; five days ago, something changed...

What did you change five days ago? Did you install anything, like an
application or an update to Windows, Office, or some other application?
Have you reset your default mail program in Windows to be Outlook?
--
Brian Tillman [MVP-Outlook]