Solution found here https://alanedwardes.com/blog/posts/force-refresh-rocket-chat-ssl-certificate/
go to where the SSL certificates are stored by Caddy (I dropped into a root shell with sudo su - first):
$ cd /root/snap/rocketchat-server/current/.caddy
And then renamed the acme folder, which contains the SSL certificate + key:
$ cd mv acme acme.old
Finally, I restarted caddy:
systemctl restart snap.rocketchat-server.rocketchat-caddy.service