unclear bug

The bug is fixed with patch 1.41. It's small – it consists of a single executable file. You can simply download a cracked version. I won't tell you where to find it, as that's prohibited. However, your save files will likely remain intact. If you don't have a copy of the previous version, the only way to revert is to reinstall the game. After reverting or installing any patch other than 1.41, you'll need to start the mission from the beginning through the campaign menu. After installing 1.41, I haven't experienced any crashes. After installing 1.5, I replayed the 1st and 3rd campaigns and then returned to the 5th. In 1.5, training and necromancy have been significantly changed.