Boris K
Auto-translated
If you load someone else's save file at the beginning of a mission, the game will likely crash at the very end of the mission. As a result, the next mission will be unavailable.
You need the campaign.sav file. It stores information about completed missions and hero progression. It's a very small file. Most problems with being unable to access the next mission are due to this file. Either it's corrupted, or a save file from another computer without this file is being used.
The file name might be slightly inaccurate, as I don't have the game on hand. For the mod that allows you to play through all campaigns in version 3.0, it's called CustomCampaign.sav.