Many thanks to Torur for the input as well, but no - on-line synchronization has nothing to do with it, as neither one of us has ever played the game on-line. Quite the contrary, I'm fairly certain the whole ordeal is strictly related to the fact that we are playing off-line and some issue is occurring every time a saved game file gets generated, causing the system to automatically delete it after the application is terminated. What I can't get my head around is why that atrocity starting happening only after we began mixing campaign with skirmish saves and why it is happening only 80% of the time.
The only solution I was able to figure was re-installing the whole fiasco of a game, so my husband is going to start the campaign from scratch and I'm going to lay off Heroes VI at least until he is finished with it, to avoid any interference with his saves. Hopefully by the time he is done, Ubisoft is going to come up with a decent patch, addressing some of the more critical issues and make the game at least semi-playable.
It's back to Heroes III for me, while waiting for LotA's final two installments. Only time will show if I'll ever come back to VI at all. No matter how hard I tried, I really couldn't bring myself to enjoying it anyway, playing it always felt more like a chore than recreation to me. I admired the beautiful graphics and the design mechanics of the game seem to have had the potential to make a truly decent sequel, but the state the game was released into was truly horrendous and thoroughly ruined the whole experience for me.
