HoMM3 HD bug reports.

Critical errors started appearing after the latest update (SOD):
1. The game crashes if you open the zone where you moved around the day before and then replay the turn.
2. The opponent's client crashes when they select a hero.

SOD was last updated on 12/04/2018.
Is this your first game since then?

If no one else is experiencing these issues,
and you're only encountering them in one specific game, it means the map/save is bugged.

In any case, and especially in your case, we need daily save files with an indication of which day and under what circumstances the critical error occurs.