Issue Summary

When creating an application link between a Confluence Cloud and a Jira Server instance that uses a Let's Encrypt certificate, Confluence won't be able to connect to the server instance and it's not possible to create the ink.

Steps to Reproduce

1. Try to create an application link between a Confluence Cloud instance and a Jira server one using a Let's Encrypt certificate

Expected Results

It should be possible to create an application link

Actual Results

The following error is shown in the UI:

And this error is logged internally:

com.atlassian.applinks.spi.manifest.ManifestNotFoundException: javax.net.ssl.SSLHandshakeException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target

Workaround

At this time, the available workaround is to use a different certificate.