For anyone else with this issue, here's what the problem was for me and the very easy solution. (Thanks to @luceos for help verifying that my database was NOT the problem.)
The issue that php flarum cache:clear fails silently if it runs into permissions problems when deleting cache files. On my server, Flarum saves cache files using the www-data user, which php flarum cache:clear can't delete when I run it under my regular SSH user, leaving the cache only partially-cleared and my Flarum install non-operational.
So if you run into this problem, try simply running sudo flarum cache:clear (or whatever command that is required to run php with elevated permissions on your OS/distro), which solved the problem for me and got my forum working again.
