No sooner did I post, when I found the answer. As much as I'd like to take credit, I found it here: https://forums.adobe.com/thread/423183 – thanks to Ken Grace (although he was resolving it for CS4). Apparently this has been an issue since CS2.

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Have you tried going to Windows Explorer> Folder Options > File Types

select INDD

Advanced > Edit > Open action

and see that it contains

“C:Program FilesAdobeAdobe InDesign CS4InDesign.exe” “%1”

I'm guessing the CS4 location, because I'm still on CS3.

The quote marks round the “%1” are important. I have seen them omitted after going through a standard use of the interface and that won't work.

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