The Let's encrypt certificate for https://stats.export.softwareheritage.org/ expired two days ago.
This impacts the Software Heritage web application as history counters data are retrieved from that place.
Consequently, the archive evolution graphs are no more displayed in the homepage.
There was a bug in the (manually modified) Let's Encrypt config for this domain, which was silently ignored by certbot (nice), and prevented the renewal to trickle down to puppet.
There was also a (minor) bug in puppet where a certificate change wouldn't restart apache.
I've manually triggered another renewal of the certificate (certbot renew --cert-name stats_export --force-renewal) and the next puppet run deployed the new cert properly.
FWIW the expected automatic renewal happened and the updated cert has been deployed on the subsequent puppet run.