This is the response object from a successful submission. The client should be rendering it into the message thread. If the user is ending up on a page that is showing just this data it sounds like the action isn't firing off as an ajax action and it is instead reverting to a form post.

Where is he sending the message from? (/conpherence/, /conpherence/$id/, /conpherence/ -> new message, from someone's profile, etc?)

Do other ajax-y Phabricator actions work?

Should try clearing the browser cache just to make sure its nothing silly cached.

I don't know what ajax-y Phabricator actions are. The guy can comment commits and audits and everything just fine, though. Will check the other things you mentioned tomorrow

The Problem was that the user had a script blocker activated. Is that something that must be supported nonetheless?