I see the same error. You must contact info@ycombinator.com because this threads sometimes go unnoticed.

[I'm not 100% sure that info@ is the correct address for technical problems, but it's listed as the official YC address so they will know how to forward the tip to the technical team.]

Chrome warning about YC blog has been resolved, I can see SSL certificate is activated for another 3 months from 18 June to 16 September

The certificate is expired, it may be an issue with the automated certificate renewal process.