Typically this is caused by an issue with the license on the machine waiting for some sort of input on the server that isn't displayed on the screen where the application is running.
If the license being used is an evaluation license, the EvaluationMode must be converted to either Watermark or Timeout to prevent the dialog box from being displayed.
If it is a runtime license, check to make sure that SetSolutionName and SetSolutionKey are being called in the correct location.
If the license is a development license, ensure that SetSolutionName and SetSolutionKey are not getting called.