Walkthroughs for custom maps.

I'm playing Paragon in Heroes of Might and Magic. There's a bug – instead of the dungeon where Irvis should be, there's a mill. How can I fix it?

First of all, you need to read what the author wrote before starting the game: these texts are directly integrated into the storyline events of the first day. You can read them in the editor, or I'm attaching a file where all of this is compiled together.
So, read in the FAQ section, the first question:

Q: Which version of "Heroes" should I play the map on?
A: The map was created for SoD/Complete Collection, the latest version. For a comfortable gaming experience, it is highly recommended to use the HD mod. The map will most likely be unplayable or completely unplayable on WoG/HotA

You even managed to "Wogify" the map: when "Wogifying," some of the map's objects are replaced with others according to the WoG settings. Therefore, no storyline RPG maps should be "Wogified" under any circumstances! Otherwise, you will almost certainly encounter unsolvable problems!
Now, just launch the "PURE" SoD/Complete Collection, which you definitely have: this is the Heroes3.exe file. If you have installed the HD mod, launch it and in the settings, choose what to launch. And start the walkthrough from the beginning (New Game - Single Player - ...).
And one more thing: if you don't like to read, then this map is not for you. There is a lot of text, and you didn't even "manage" to read the author's instructions.
And if you want to play specifically with WoG features, then you need to take either maps of the WoG version (there are plenty of them in the directories) or purely strategic maps (there you can usually play for any color and the win condition: defeat everyone).