MQTT connection issue

MQTT extension does not connect to mosquito MQTT server. I´m using the thingworx server instance provided as part of the vuforia pilot program, but I get everytime only the message: Unable to connect to MQTT in [MQTT Test].

I followed the setup instruction I found in a PTC forum and also did test with MQTT_spy to make sure that the MQTT broker ist running.

All works fine, only Thingworx server will not connect and ends up with the message: Unable to connect to MQTT in [MQTT Test]

Any ideas what I´m doing wrong ?