HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

The first signs

What's going on? How are you managing to get these errors? I can't seem to reproduce them...

The "swallows" aren't the first. The code that caused the error hasn't changed since version 3.0 beta 1.
According to the report, the game crashed after a left-click was made in the town window. Where exactly did you click?