This error occures, if there are "unknown letters" in the path/mapname.
In the path/mapname there should be no other latter than 0-9 and a-z. Especially some special letters, like ä, ö, ü, ß cause this problem. Quite often users have such a letter in their username (username of windows) and therefore they can't load the map.
What is the whole path to your map folder? (as an example, it is for me: C:\Users\Johannes\AppData\Roaming\My The Lord of the Rings, The Rise of the Witch-king Files\Maps)