But considering that everything works fine after restarting the map, the issue is likely not that the class is specifically prohibited on your computer. "
For everyone whose classes aren't leveling up from the start, you should check this file!!!

It's absolutely irrelevant what's in this file for the players because it's read during the game's startup and isn't used afterward. If a class is prohibited, this prohibition is "hardcoded" into the save file during its generation, meaning it's important how the checkboxes were set when the save file was created.
Also, I'd like to note that it's preferable to use Alt+click on the hero's portrait in the player's hero list to obtain a class, rather than the "h" button (which doesn