Hi,
thank you for your quick reply.

I have extended the configuration to log on level debug.
It gave me the hint:

Caused by: com.ibm.mq.MQException: JMSCMQ0001: IBM MQ call failed with compcode '2' ('MQCC_FAILED') reason '2400' ('MQRC_UNSUPPORTED_CIPHER_SUITE').
at com.ibm.msg.client.wmq.common.internal.Reason.crea teException(Reason.java:203) ~[com.ibm.mq.allclient-9.1.0.3.jar:9.1.0.3 - p910-003-190717]

I deactivated the cipher mapping by using:
-Dcom.ibm.mq.cfg.useIBMCipherMappings=false

That solved the problem and I got:
javax.net.ssl.SSLHandshakeException: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderE xception: unable to find valid certification path to requested.

So I had to provide a trust store with a valid certificate for the SSL handshake as the IBM-MQ I used is set up with certificates based security in contrast to my ActiveMQ Server.

Setting the following properties will do it.
-Djavax.net.ssl.trustStore=path/to/truststore -Djavax.net.ssl.trustStorePassword=...

So, it was definietly a java security issue, not a JMS extender issue at all.
Keep up your good work.