Google's mod_pagespeed is a great open source module for Apache, unfortunately, it's default settings breaks concrete5 when enabled.

mod_pagespeed tries to re-write urls to make them more efficient, but sometimes it rewrites urls it should not. To disable mod_pagespeed's url re-writing you can simply add this to your .htaccess file:

ModPagespeedDisableFilters trim_urls

Alternatively, if you want to disable mod_pagespeed altogether, you can add this instead:

ModPagespeed off