I just want to share this hoping it might save someone from going crazy :o)
First the background:
I updated Backdrop from 1.25.1 to 1.26.1 and modules accordingly. It all works perfectly on my dev server, so I went to deploy it ... and it doesn't work on my production server.
PHP versions are not exactly the same, but I think close enough - 8.1.15 vs 8.1.25. Also, the 1.25.1 site was also running on PHP 8.1.25 and it works. They are both Apache servers. Status report doesn't show any errors.
I did clear caches ....
I don't think it is because of the specific theme because I still have the same problem with Basis and Seven. It shows as a basic HTML site without the admin bar at the top, no tabs, no columns, no dropdowns, no image resizing, not responsive, etc.
Also, using the updated DB with the old site directory works, so I think that the problem is in the files / dirs not the DB, but I can't see how because it works on the dev....
And it turned out to be ... permissions. My hosting co no longer sets the owner or group on files and directories under public_html to belong to the Apache server. So everything has to be world-readable, unlike my dev server. Once I did:
chmod -R o+rX files/
everything worked. :facepalm:
Thank you for sharing. Hopefully this will help someone else in a similar situation.