How to connect to Youtrack-Server with SSL?

In the connection setup I add our server-URL (which starts with https), user and password. Connection fails with following message: SSLHandshakeException: PKIX path building failed: SunCertPathBuildingException: unable to find valid certification path to requested target

If I just start creating a workflow with a rule offline and I do an export, the rule does not appear in the export file, so offline working does not seem to work

What can I do?