This can happen when Zaurus cannot access the camera feed. It usually happens after an unplanned disconnect. The camera is still reserved by the OS preventing Zaurus from using it despite the OS thinking Zaurus is still using it.
A simple reboot is usually the easiest way to solve this. Sometimes waiting for a few minutes can also work.
It could also be caused by inactive streams kept alive by the server. In these rare cases it is best to continue the consult with another device. Since Zaurus is multi-device, you can be logged in on several devices at the same time.
The inactive streams will be removed automatically after a few minutes.