It's hard to be certain without seeing information about the certificate behind the warning. Still, I expect the OP's issue is because their system doesn't have support for new Sectigo Subordinate CA Certificates, including Sectigo Public Server Authentication CA DV R36. LiveATC started using a certificate signed by that last month.
Searching the web for "Sectigo Root CA migration" should turn up a number of documents about the switch, although mostly from the point of view of site administrators. As an end user, one needs to add the appropriate certificate to your certificate store. I did that last month on a RPi system by downloading a copy of the cert to /usr/local/share/ca-certificates and then running update-ca-certificates.
