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This looks like a problem with ISPConfig. We recently helped someone with a similar pattern of certs getting revoked. See: Certificate keeps getting revoked with "Cessation Of Operation" - #10 by frank42hh
They found how to stop ISPConfig from revoking their certs although they still did not understand all the reasons it started happening. That is probably best to ask of ISPConfig support or their community. A quick check on their community found this: certbot keeps revoking certificates | Howtoforge - Linux Howtos and Tutorials