By coincidence I found that de the root cause is the feedback widget. When added to the homepage, this happens immediately, when before that change starting the server locally succeeded. Removing the feedback widget from the homepage again does not solve the problem anymore. The modeler then still cannot start the server. The eroor details are empty. The listener is indeed not started when I check the list of listeners and port on my system. After that, all my other MX apps won't start either, so it's definitely messing something up in my system. Even trying to start the server on an any other port, like 8081 and 9091 wont help. Restart of the PC does not help either. Only after a crash of the system the problem seems to be solved. ==> Does anyone know how I can "clean" my PC whithout forcing it to crash?