The computer has 64GB of RAM so that should be enough. The desktop is 3840x1200 (which is 2 monitors of 1920x1200) and the 4 desktops are also 3840x1200. The strange thing is that it worked on my old W7 system with 'only' 32GB of RAM.

Could it be that the computer you're running Dexpot on doesn't have enough RAM to handle all of the desktops at once? When you're saying you have 4 desktops and 2 monitors, how are they organised? 4 desktops duplicated on each monitor, 2 per monitor, 4 on one monitor and something else on the second?

My Windows10Pro (Version:10.0.18362 Build 18362) PC has 2 monitors (1920x1200 each) and I have configured 4 desktops. For each desktop I've selected different fixed desktop colors. For desktop 1 and 2 this works without any problem, but desktop 3 and 4 appear with a black background. Tried different colors but did not work. In the 'Configurations Desktop' > 'Background' form I've made these selections for all 4 desktops: Multi-monitor background mode is set to 'Windows default', Path is empty, Alignment is set to 'Stretch' and 'Desktop color' is set to a predefined color (different for each desktop).
Any idea how to get the selected color on desktop 3 and 4?