I'm assuming you need to keep your live site online and don't want to shut it down?
Use JoomlaPack to create a backup, then install on your local machine in XAMPP. You should then have an exact copy of your live site running on the local machine for you to fiddle with.
Then you can go through every plugin and every module, disable them one at a time and test, if all are exhausted yet problem still occurs then try 1-by-one "reverting" the site back to a fresh Joomla install (i.e., uninstall a Component; Change a Global Setting, etc). And via said process of elimination you will eventually find which 3rd-party extension or non-default configuration change was causing the problem - then we might be able to update JoomlaComment to make sure this doesn't happen again, or contact the other party involved about their bug (or even fix it for them).
