I have a website using the concrete5 CMS, and I discovered that it does not work with Google's mod_pagespeed for Apache.
After installing it, it's not possible to add new pages, you'll get an error 404.
To disable it only for concrete5 websites, just write this in the .htaccess file:
<IfModule pagespeed_module>
ModPagespeed off
</IfModule>
In my opinion, the problem it's in the trim_urls filter, so you could just try to disable that.