Old OS like Windows 7 or Android not getting root certificate updates any more?
Chrome browser that is out of date?
Some firewall or ad blocker proxy trying to intercept communications?
The lack of any details makes it hard to suggest a path to pursue.
Cloudflare is serving up a certificate chain for ChatGPT that has RSA2048 and SHA-1 with RSA Encryption v3; Elliptic Curve 256; and ECDSA with SHA-256, with earliest expiry 23 Oct 2024.
I went as far as infesting a Windows 11 box with Google Chrome download, and am chatting away.
Cloudflare has many point-of-presence gateways, possibly with several OpenAI datacenters behind their firewall, so replication of this issue may be hard. Hopefully just a passing occurence affecting very few.
A different browser might give you a different view of the internet issue.