Daniel was talking about sh404SEF, not your custom 404 page. It's probably the most widely-known Joomla extension among the community. You are obviously using some kind of SEF URL generator component, such as ArtioSEF, we can tell. sh404SEF is the only one we really test (it's the most powerful one) so it might be that causing it.
Do you have a custom .htaccess file? If so, try going back to the default one that comes with Joomla.
Do you have Legacy Mode enabled? Try disabling it.
Is your MySQL database in utf8? If it's not, let us know. If it is, then that's good.
What else I can suggest, try disabling the following Joomla extensions and testing the comments after one by one:
- PhocaDownload
- mod_swmenufree (could be JavaScript conflict)
- mod_gtranslate (this one could be it, it seems to intercept POST data which JoomlaComment uses on Comment submit)
- mod_hot_image_slider
- mod_stalker
- mod_taf (I can tell this one especially is poorly coded and might cause other errors
I cannot figure out why you have three FavIcon declarations in your page source either. Plus, there is a lot of inline CSS... but that's not important. Just an observation.