Hi @afagund
Chrome requires that certificate authorities like Let's Encrypt submit issued certificates to a number of Certificate Transparency logs in order for the certificate to be considered valid & trusted: GitHub - GoogleChrome/CertificateTransparency
One or more of the Certificate Transparency logs that Let's Encrypt is using to meet the Chrome requirements is not trusted by Chrome 66 (maybe it didn't exist at the time Chrome 66 was released for example). The set of trusted logs changes release-to-release (some may be added, some may be removed). Edit: In particular, Chrome 66 is missing the "Yeti" family of logs that we submit to. It was added in Chrome 67.
There are a multitude of reasons why users should always be running the most up-to-date version of their web browser. This is another good one 