Appears problem resolved by creating unique project for each backup set. Previously, we created a unique client key for each backup set. We now create a new project for each backup set and then a new client key within that project. So far so good for past week or so with no errors.
representative configuration screen shot at this Google Drive link.
Thanks for working with us to resolve. May be worth mentioning in help file &/or clarifying the on-screen instructions to this effect (if we're supposed to create a new project and not simply a new client key for each backup set within an existing project, we missed it).
Cheers! & thanks again!