I was upgrading troops after battle, in the window, where you take prisoners etc. I had captured few looters, and freed like 5 of empire peasants which I recruited. Then I proceeded to upgrading troops, I upgraded imperial recruits to archers using ctrl + lmb, then I moved them up a notch or two, still holding ctrl, then I tried to upgrade other troops, still using ctrl + lmb combination. This is where game crashed.