VNC4 giving me black screen when connecting

Hello. I have vnc4 running on my XU4 and I can connect normally after a reboot (using VNC Viewer in Windows). However, after some time an attempt to reconnect to the XU4 results in a black screen with a small square instead of a mouse cursor. It appears I’m connected but I’ve lost the GUI.

Any ideas? Should I try a different remote desktop app on the XU4?

Thanks all