Ankorchik
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Good afternoon. This is a long-standing problem in Heroes 5; the game cannot consume as much RAM as it needs, and the computer configuration has little to do with it. During testing, I sometimes couldn't load saves when I loaded them directly during a battle, but when I re-entered the game, they loaded normally. To ensure that this map and many others (very large in size and/or very densely populated with objects) work stably, you can try installing this patch: patch. I took the link from a topic dedicated to this very problem; there, another solution is also posted: topic.
I haven't tried this solution myself, but I hope it helps.