I've noticed something odd going on...
When a user deletes their account on the site, if you've had a conversation with them, the conversation remains. Obviously, it may not be best practice to delete the conversation, but unfortunately, it says the user is online, and if you click the avatar (which is replaced with a generic head on grey background), you get a 404 error because that profile no longer exists.
What seems to be needed here is that the avatar link is disabled if the user no longer exists, with a message that says "this conversation is now locked due to the user you were chatting with having deleted their account". Also, the status should not show them being online, and perhaps needs to be replaced with something like a red X saying "Deleted".
Dave