Safari can't establish a secure connection due to certificates that are "not standards compliant", reboot fixes it

Hello,

Starting around June 2020, I've been having issues establishing secure connections in Safari & other apps that utilize MacOS' certificates. After leaving my Mac running for about 18 hours, Safari gives an error saying it's failed to establish a secure connection, & clicking into the details of the certificate show that it isn't able to validate them due to the certificates not being standard compliant. This doesn't happen on all websites, just some.

Steps I've done to try to resolve (but they fail to permanently fix. Some resolve the issue for a while, but after some time the issue pops up again).

1. Add custom certificate preferences to allow certain root certificates. This sometimes resolves an issue for a while for a particular site, other times the issue persists & even if I force allow all the certificates in the chain from root all the way to leaf it doesn't establish a secure connection due to the same reason.
2. Delete any custom certificate preferences. This helped for a week or so but then the issue came back.
3. Re-install Mac OS Mojave (not a clean install).
4. Uninstall all AntiVirus/AntiMalware apps.
5. Uninstall all AdBlocker apps.
6. Disable LittleSnitch.
7. Disable all startup items (e.g. any apps that I've installed to launch when I start up). This helped for a few weeks but then the issue started again even without any apps launching at startup.
8. Disabling IPV6.
9. Setting my DNS to 1.1.1.1.

In general, all of the above don't permanently resolve the issue. I keep having to restart once or twice a day for the secure connections to establish.

Any thoughts on what it could be that's going wrong, how I could analyze the issue further, or resolution suggestions?

Thank you!

Safari: 14.0.2 (14610.3.7.1.10)

MacOS: 10.14.6 (18G7016)