Let's encrypt updated their certificate chain, therefore client chrome browser got cert error. Firefox is ok. Looks like this chain is hardcoded in /usr/libexec/webmin/webmin/letsencrypt-lib.pl, I corrected this file with https://letsencrypt.org/certs/lets-encrypt-r3-cross-signed.pem insteda of x1, then chrome browser did not show me a cert warning.
UPD: Sorry, I replaced r3 to x1, not r3 to x1.
But after I rolled back changes in letsencrypt-lib.pl my chrome did not show me any warning. I'm not sure is it a chrome or webmin trouble.
What was the exact error you got in Chrome? Also, was this for a cert that was originally issued by Let's Encrypt?