I just explored my logs and it shows the following:
[2020-06-05 09:09:00] WARN Request to “https://some/url” failed because of: CERT_HAS_EXPIRED.
can anyone tell me please what certificate is expired?
This request is triggered by “member added” webhook to Netlfy’s function.
https://some/url has an expired https certificate 
Interesting, https://some/url can be opened in a browser using HTTPs without any issues, do you know why that might be the case?
It could be that the certificate was expired for a short period of time ~ if you can trigger the webhook again, does it still fail?
yeah, it’s been happening for a week now
I’m not entirely sure outside of that - the only thing I can think of is your server’s time is out of sync, but if that was the case, your posts would also probably have issues ¯\_(ツ)_/¯
Having the same issue. Webhook I’ve been hitting for years all the sudden is erroring, but not in browser.
A frequently-used intermediate certificate expired recently, your error could well be due to that. Browsers are better at dealing with expired intermediate certificates than embedded TLS libraries like you have in node.js. I’m not sure what the fix is but hopefully it may point you in the right direction