SAP CPI Open Connectors Adapter - PATCH-Method for specific objects isn't working

The Integration Scenario is the following:

The Receiver is OpenConnectors. From there it goes further to Jira. An existing issue (object) in Jira should be actualized.

The Adapter Settings:

In the Content Modifier the Content-Type "application/json" ist set. By executing this Scenario with a "GET", it is working without Problems. But other commans like PUT, PATCH and POST aren't working. The error I get is:

com.sap.it.rt.adapter.openconnectors.exceptions.OpenConnectorsException: HTTP operation failed : 400

Any idea why the GET Method is working, but other Methods like PATCH aren't?

Regards

Philipp