Invalid certificate Mac or Safari won't open any website
I've been trying to solve this problem.
Finally answer is to Mac sure date and time are correct.
Took me 4 weeks to figure that out by deleting wifi drivers, etc.
Anyway, solved.
Check this web for a checklist: https://appletoolbox.com/safari-cant-establish-a-secure-connection-how-to-fix/
MacBook Pro 13″, macOS 10.15