I looked in Apache’s error_log file and noticed multiple lines like this:
an unknown filter was not added: DEFLATE
After thinking a bit I recalled that I added the following line in the .htaccess file in the root directory for on one of my domains, but never tested it properly:
AddOutputFilterByType DEFLATE text/html text/plain text/xml
The fix is easy: enable mod_deflate. On Ubuntu:
$ sudo a2enmod deflate Module deflate installed; run /etc/init.d/apache2 force-reload to enable. $ sudo /etc/init.d/apache2 force-reload * Forcing reload of apache 2.0 web server...
Why did I add the above filter line in the first place? It was suggested by YSlow.
Update Another kind of fix is of course to remove AddOutputFilterByType.