My Comodo/Sectigo SSL certificate shows as invalid in some browsers

If your domain's SSL certificate shows as invalid in some older browsers (or another software like email clients), you may have forgotten to install the intermediate certificate on your server.

You can run SSL test at https://www.ssllabs.com/ssltest/ and see "Extra download" label in "Certification Paths" section.

Modern browsers can download intermediate certificates on their own, but some older software requires them to be provided by your hosting server and show an error if they cannot verify the chain of trust to the root certificate.

Simply add an intermediate and root certificate to your server so that any software can verify the certificate chain up to the root.

More - https://www.sectigo.com/knowledge-base/detail/Sectigo-Public-Intermediates-and-Roots/kA0Uj0000003eovKAA

For Positive SSL you can use:

Intermediate - RSA: Sectigo Public Server Authentication CA DV R36 (https://crt.sh/?d=4267304690)

Root - RSA: Sectigo Public Server Authentication Root R46 (https://crt.sh/?d=4256644734)