In a specific dynamic the following happen.

Let's say we have 2 campaigns (11 & 12) that I can ambush.

If I click on 11 and select 5 heroes, but decide to close the ambush pop up. I then click to ambush 12, select 5 heroes (some of whom were selected for the ambush not done on 11), then validate my ambush. If i then click on 11 to ambush, as one of a hero is missing the full screen becomes blank (full blue, without the pop up, nor the menus, ....)