Hi @radjr,
This is a problem with an expired certificate on your server; you’ll need to update the server’s certificate list. This will affect many sites using Let’s Encrypt certificates, not just PKP’s. See for example:
https://blog.devgenius.io/rhel-centos-7-fix-for-lets-encrypt-change-8af2de587fe4
Regards,
Alec Smecher
Public Knowledge Project Team