Hey Bruno,
It actually turned out that the problem is not sh404, but the joomla cache (I guess that when you turn off sh404, it clears the cache).
CComment is not going to work with Joomla's cache turned on.
The problem is - Joomla is trying to cache the output of the whole page. Which is theoretically fine, but it is fine only for static content & not dynamic one. Comments are dynamic -> I need to know the user that is currently browsing the page etc.
In your case the issue is that joomla is creating the same cache for the menu item, no matter what the url parameters look like. For example, when you add &_escaped_fragment_ to the url we render the comments for the search engines as per specification
developers.google.com/webmasters/ajax-cr...docs/getting-started
Now the thing is -> when you go the url without the &_escaped_fragment_ then joomla will serve the cache that it generated for the page with _escaped_fragment & those 2 are not the same.
CComment is optimized to work without joomla's cache (and we are constantly committed to improving the speed of the code).
Regards,
Daniel