Clean URLs work*, but not when I submit forms? I have a local dev site configured (Ubuntu 14.04.1 LTS), and I copied a production website to play with there. Everything appears to work great. Clean URLs seem to work and I can navigate the site. It's all wonderful... until I try to login.

When I login at /user, submitting the form returns me to the login as if nothing happened. I was able to login by visiting /?q=user .

After that, if I attempt to clear the cache under performance settings, I get a similar result where the performance page loads as if nothing happened (clearing cache manually via mysql doesn't help). If I disable clean URLs altogether, all is well.

I tried replacing the .htaccess file with Drupal's default to prevent special config issues (no effect). I didn't see anything set as $base_url in settings.php (I'll try setting that to see what happens next).

Any thoughts on why a site with clean URLs enabled seems fine until POST data is involved? Thanks for the feedback in advance!
I think you need to enable Apache's rewrite module using a2enmod rewrite or with sudo if you get permission errors and added the Apache directive AllowOverride All for VirtualHost.