This is probably more an operational issue than a bug.
Let’s Encrypt appears to have performed a mass revocation of a subset of their certificates; certificates for forum.wolfery.com, and mucklet.com appear to have been affected. I am only able to send this by virtue of the --ignore-certificate-errors argument to Chrome, which allows me to bypass it. Without the argument, I get either ERR_FAILED from Chrome and edge, or an out of date cached copy of the page; a shift-reload yields NET::ERR_CERT_REVOKED.
One reference (such as it is) to what appears to be going on appears to be at Let's Encrypt to revoke about 2 million HTTPS certificates • The Register