There is a JS error coming from RokBox. JS breaks when you start typing a comment.

There is now a server-side error when you hit the "submit comment" button which is not shown on the page because the request is triggered by JS.

Some plugin is intercepting the POST request (to store the comment) and it breaks the output by Joomla.

I have a feeling you may have installed this plugin styleware.eu/store/item/67-performance-booster-for-k2 on your site at some point in the past.

If you don't find anything relevant, switch error reporting to maximum in Joomla's Global Configuration and let me know.

Unfortunately it's not easy to debug on my end. Some plugin is using the "Services_JSON" class, which is obviously not loaded from any file. K2 *used* to carry that file, but that was years ago and it's been removed as it was not needed for a very long time already.

Perhaps some other K2 plugin utilized that file and if you recently upgraded K2 it was probably deleted.

In any case, scan your site for any other K2 plugins. If you have a local copy, search if the string "Services_JSON" is included in any file.

Works just fine to me. I just added a comment here: jakadibio.com/component/k2/notre-microbiote-c-est-aussi-nous?Itemid=155

I used Chrome and I just used Firefox as well, without any issues.

I also checked for JS errors and couldn't find anything.

So perhaps there's a conflict with some Firefox plugin or something.