Hi Felix ,
As standard procedure , after enabling(or disabling) the Messaging connection alias , you need to restart the Integration server for the changes to take place.
In your case looks like the Broker Messaging connection was disabled , when IS started , someone would have enabled it. If you enable(or change the state of messaging aloas) it then you need to restart the IS server for it to connect successfully to the Broker.
A simple restart of the IS will suffice your need.
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