After a certificate expiration, a new https certificate is usually built automatically. Prosody is still set to use the old certificate, because there's still a copy of the previous one in its config folder.
How to update the certificate on errno64.org. Replace the domain with yours. You can set a custom filename for the certificates in /etc/prosody/prosody.cfg.lua:
cp /etc/ssl/errno64.org.fullchain.pem /etc/prosody/certs/errno64.org.crt
cp /etc/ssl/private/errno64.org.key /etc/prosody/certs/errno64.org.key
Then, restart prosody server. rcctl here works like systemctl:
rcctl stop prosody
rcctl start prosody
For the sake of automation, you could run these commands in a cron job after certificate renewal.