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!
Sometimes, when migrating servers or copying data to a development environment for example, one needs to take a copy of an entire mysql database. Here is how you do this via the command line on a redhat/centos server: