A VNC source will show a white screen during one of two situations.
While initiating the connection and application, the screen will turn white. If there are no issues with the connection to the source, the feed should start after only a few seconds. Otherwise, you will be presented with either a white screen or color bars.
If the white screen persists, verify the IP address and credentials of the VNC source.
The VNC source can show a white screen if there is no activity whatsoever on the desktop being shared to Userful. VNC operates by transmitting differential pixel data, and if no pixels change, then sometimes no data is sent initially.
If there is any kind of video activity on the desktop- even a clock or blinking cursor - then you won't experience this.