We are trying to help but you do not give us much info.
I think you have an old CA Certificates root store. An updated package was created last Sept to address problems that occurred when DST Root CA X3 expired on Sept30. See this topic. I know the title says RHEL/CentOS6 but info on RHEL7 is there too.
One way to confirm you have an old root store is with this:
grep -Ei 'ISRG|DST|R3' /etc/pki/tls/certs/ca-bundle.crt | grep '#'
Let us know what that says
Also, please let us know if that SSL Checker website said your cert was ok. Thanks