After playing through WhiteBeard's save and experiencing no issues, I've concluded it's more of a hardware/software issue.
If anyone who experiences this issue could post their PC specs, nVIDIA control panel settings set for battle_realms_f.exe and the resolution of the game you play with (for example 1024x768, stretched to 1920x1080 or true 1920x1080 with FOV patch applied), I would be grateful, and that would surely help to see what you guys have in common that possibly triggers the crash. Video of me playing through WB's save.
The only building that was left to destroy (and that I wasn't able to find for the next 10 minutes of the game) was a Vitality Garden. After it was destroyed, the mission was completed successfully.
I can suggest dgVoodoo2 as a (temporary) workaround:
Talking about dgVoodoo2 in particular, skip the widescreen patch or install it as well, let me know if it helps.