5a693e87_Plus__known_issues____RealVNC___Workaround
[ID] 158 [Description] The host computer may disconnect VNC Viewer Plus when a dialog box is shown or the window is minimized. [Status] Under consideration. [Workaround] 
Perform an operation (causing a screen update to occur may prevent a timeout).
VNC® Viewer Plus (known issues) - RealVNC®
Workaround