Damn, either my Heroes game is messed up, or my computer is acting up. There are no text windows, side quests can't be completed, and missions can't be finished. I snapped on the third mission when, after accumulating 10k gold, I couldn't open the merchant's shop. I exited the game, went back in, and the game crashes halfway through loading. I don't know what to do; I couldn't complete the campaign, and I doubt I'll be able to play new ones until I find out the reason. I'll try creating a new Steam account on a new device and buying Heroes again there. It doesn't work properly on the pirated version either.
And this is happening with all the campaigns I've played – everything works fine for everyone else, but it's a mess for me.