Howard Miller

That's a PHP class, so it implies that you don't have the Intl PHP extension installed or it isn't working.

So, to be clear, all you've done is *migrated* you installation to a local machine and that didn't work? You've not tried to upgrade at all (yet).

Whatever happens, your next step is to enable Debugging

The latest version is 4.2 and you cannot directly upgrade from 3.7. You would have to first upgrade to 3.11 and then to 4.2. You also have to *carefully* read the release notes and consider versions of server software for each step (especially PHP).

https://moodledev.io/general/releases/4.2
https://moodledev.io/general/releases/3.11

So, the next question is, what did YOU do that resulted in this error?

True - but I would add in an extra step. I would test the whole process on a separate set of VMs entirely separate from my production site. This includes all the server upgrades, the main Moodle code, all my plugins and any custom code. The whole lot. When I'm 100% happy I'll recreate all this on the live site.

Working, as I do, in a large University. In real life, this takes a small team a number of months planning such that we end up having the live site offline for the shortest time possible.