Thanks for the responses.
Did the following;
Deleted the save folder
[rawdisplay]
dpi=299
[interface]
resolution_dir="2560_1600"
Alas no fix.
The issue is that initially all is ok at first startup but after 1, or more, position updates the upper righthalf is black.
Changing the sky image is something that doesn't fit the symptoms?

Regards, Kees.