I have the exact same problem, spent several hours debugging my site and found that the problem is caused by the "System Artio JoomSEF plugin" disabling this allows CComments to function.

I am running the latest software versions:
Joomla 3.4.8
Artio 4.6.5
Ccomments 5.3.5

I have tried setting Artio extensions for CComments to "Don't SEF" as the handler but there is still a conflict. Please advise as a matter of urgency?