F-15 radar exported viewport white

I am using multiple monitors (see this thread: http://forums.eagle.ru/showthread.php?t=148172 ) and I encountered the following problem while flying the F-15C:

The exported viewport for the radar screen that I placed on my third screen shows a white window on mission start, where the radar image should be, with some red lines on it that look like the green lines on the radar screen, just a bit distorted. Then the lines fade away, and it takes about 3 seconds for the viewport to become completely white. It stays that way regardless of radar operation.

The exported viewports work fine in the A-10C, so this is not a general problem of my setup I think.