The message may also occur when there is no streaming type application running in the background. Usually, you encounter that error when you have other applications, which by default or not, take advantage of all the resources found on the computer. This means that you have other application, other than a streaming one, which is trying or it has locked the camera or the video device.
Try and close all the unnecessary running application and see what happens when you start Dell Webcam Central. Most likely, this is the problem.