If anyone facing this issue then check SSL certificate integration in your server. For me the issues was with (GoDaddy)SSL certificate installation.

Here is the solution suggested by unbounce team:

if POSTing to http, you should be able to deliver the lead without issue. The issue arises when trying to POST to https.

They ran an SSL analysis on the server domain (You can generate a similar report using this link - https://www.ssllabs.com/ssltest/analyze.html?d=hmilms.com&hideResults=on). Based on the results, it appears the certificate chain is incomplete and we’re unable to fully verify the certificate chain as there should be an intermediate certificate that is not being presented by the server.

This is likely the cross-over certificate (gdroot-g2_cross.crt) that is missing in the GoDaddy cert bundle which is needed due to how GoDaddy SSL certificates transitioned from using SHA-1 hashes (G1) to SHA-2 hashes (G2) - https://tozny.com/blog/godaddys-ssl-certs-dont-work-in-java-the-right-solution/

There is a solution included in the link above which has a link to the cross-over certificate that will need to be added to your certificate chain.

That should resolve the certificate chain and allow for leads to be posted to your webhook via both https and http.

Could you try installing the cross-over cert into the certificate bundle that is in use on your webhook server? Once that’s installed, Unbounce should be able to see the full certificate chain as presented and leads should be able to be POSTed to your webhook server again.