I recently bought a DLink-932L (Firmware: 1.09 (2014-03-14)) and ran into the same aplug.class - ClassNotFoundException that others have experienced. This occurs even when trying to connect from the same subnet/physical network as the camera.
It looks like one cause of this problem is changing the camera's HTTP Port from 80 to anything else. The code to show the Motion Detection and Live Feed Java applets appears to have been hard-coded to use port 80. I have included an example of the applet code below (taken from Motion Detection page source), which shows port 80 even though I had previously changed the port to 932.
Changing the HTTP Port back to 80 fixes the issue.
I would imagine most people leave the port at 80, which is why not everyone experiences the issue.
Dan
<applet name="cvcs" codebase="http://192.168.1.201:80" archive="aplug.jar" code="aplug.class" width="640" height="480"><param name="RemotePort" value="80"><param name="Timeout" value="5000"><param name="RotateAngle" value="0"><param name="PreviewFrameRate" value="2"><param name="Algorithm" value="1"><param name="DeviceSerialNo" value="..."><param name="SetMotionDetection" value="1"><param name="MotionDetectionBlockSet" value="1111111111111111111111111"></applet>