Magento 1.9. One page checkout not working past address stage

The checkout issue of not being able to proceed past entering the initial address is a real sod to sort out.  There seems to be a variety of possible causes almost all of which leave no easy trail of errors or, in some cases, no errors at all.  I am posting my latest discovery here after migrating a site a new server to find that I was unable to progress past stage 2 of the one-page-checkout process. Spoiler alert: it was the CSP!

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