Whenever the input data changes or the node gets a new input connection, the corresponding #configure method in the NodeModel is called. If this method runs through, the node will be configured (yellow), if you want the node to fall into the not-cofigured state (I hope that is what you meant with IDLE), you will need to throw a InvalidSettingsException during #configure. Hope this helps?
That was exactly what I meant. I changed my #configure method and everything works now.
Thank you very much!