It gets worse ... now every time I go to a desktop the icons move another 250px to the right, to the point where they disappear completely.

I've just upgraded Dexpot and a problem has arisen - there's an 'exclusion zone' about 250 pixels wide on the left of the desktop running the full depth of the screen which won't accept icons.

I can place them there, but if I switch desktops then come back, they've moved 250px to the right!

Any ideas?