A green screen indicates that you've connected to a different channel than the one used for streaming or there is an error with the camera's streaming mode. This could be an issues with the physical connections as well, but not on the application. Check the NVR is you use one and inspect the cameras. Connect manually to the camera's administration menu to see if you can playback its stream separately and in a browser. If it's possible reach support or warranty for cameras and send them back.
The UMS Multi Client might be working fine without issues.