The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel.

Hi

We've build an APIM instance within a VNet (internal mode) and using a self-signed certificate (*.abc.com) to secure the APIM endpoints (like Gateway, Dev portal etc.). We've tested that these portals work when accessing them using a VM within the VNet - just that we get SSL certificate error (expected) that we can bypass in the browser.

We have a BizTalk server on-prem that hosts an API. This API is configured as a back-end in the Azure APIM. The on-prem BizTalk server uses a different self-signed certificate (*.xyz.com). When we try hitting the BizTalk API endpoint (URL) directly using the VM in VNet and bypass the certificate validation, we get a success response (Test case 1). However, when we try and hit the same URL (configured as back-end in the APIM) via the APIM (Test case 2), it fails with the following error:

"The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel."

Here's a diagram to visually depict the two test cases:

Can someone point out what could be the issue and possible solution here?

Thanks

Taranjeet Singh