Try disabling these settings on /admin/site_settings:

• Use simple cache
• Use system cache

Then go back to Dashboard and run Upgrade.

If everything worked, change the caching settings back and run Upgrade again.

If this doesn't help and/or you get a message about the upgrade process being blocked, then check the permissions for /data directory on your server.

Permissions should always be set for the server, i.e. www-data, http2 etc

If you run something in the console as root, e.g. php elgg-cli or copy/create something in /data directory, then you may have such problems.

Always change the permissions back with this command:

chown -R www-data:www-data /path-to-your-data/

Learn this also: 1, 2, 3

Tip: use this plugin if you need to control files in your /dataroot directory.

Thanks for the quick reply, Nikolai!

Unfortunately, my problem persists.

Doing the .. disable-cache, upgrade, enable-cache, upgrade .. all seemed to succeed (no errors), but the CSS is still missing. The permissions on my data directory look fine (and files are being written in there). I haven’t really done anything at the command line or made any changes other than the few changes in the Admin / Settings. This is a very vanilla installation.

I reviewed the other links you provided, and none really apply to my situation.

My “production” installation still seems to be working (although I haven’t touched the cache settings). I’m wondering if something has just become corrupted in the “dev” installation. I think I’ll flatten it and reinstall, and then see if I can reproduce the problem. If I can’t reproduce the issue, I’ll just chalk it off to some random corruption. Will let you know how it all goes.

Thanks!