We’ve successfully made letsencrypt work for Encrify at https://www.encrify.com .
Encrify is a secure chat/IM tool with ultra privacy, targeting chat within family, among really really close friends or in small teams at work. The server is on-premise at your home or office, with a letsencrypt issued certificate, which is accepted by all major browsers, besides the encrify client.
Encrify helps you take back your privacy in chat. Give it a try and you’ll be happy that your privacy is now in your own control, not in others’ hands.