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Then it's quite strange. If this isn't a conflict between game versions, then either the recent campaign update is broken, or there's some other magical and unpredictable reason on your end.
I could also suggest that the campaign is too demanding for your hardware... but is that even possible? It only contains a few models from Heroes 5 and not the most complex (in terms of detail) landscapes in the history of mapmaking.
My computer can handle anything, it's a top-of-the-line machine, so it can definitely handle Heroes 5. And consider this: I've already completed four different campaigns. Yours, Andrey's, BlueHaven's, and this one - and in each of them, I encounter some kind of major issue, something breaks, or something else goes wrong. And even on a completely clean version of Heroes 5, I still have some bugs. I'll try reinstalling Windows, maybe that will help.