Unfortunately this was an issue with ZeroSSL certificates.
The quick fix is to delete the directory for your domain that’s stuck from Caddy’s data storage, then restart Caddy. This’ll force it to fetch a new certificate.
Since you’re running Caddy as root, I think it’ll be in /root/.local/share/caddy/certificates/acme.zerossl.com-v2-dv90/your-domain
After updating ZeroSSL with Caddy, your Firefox may still be returning an error.
The old certificate is maintained somewhere. You can work around this with Firefox settings:
≡ - Settings Left sidebar Privacy & Security
Scroll down “Certificates” Query OCSP responder servers to confirm the current validity of certificates to Uncheck
I have confirmed that Caddy, which has the same problem, can reissue the certificate by this work.
If you have a lot of domain management, a patch module is already provided. (see GitHub issue)
You can also wait for Caddy’s update if you’re not in a hurry.