| instead of / ?
That sounds wrong paths are not written in that manner

maybe they changed how teh thing gets parsed (but I HIGHLY doubt that)

this allways worked:
string mapPath = "resources\interface\maps"

Change this line to read:

string mapPath = "resources_override\interface\maps"
Now this should work:
The map path is now kept in Horizons\prefs\ClientPrefs_Common.txt Open that file with notepad, scroll to the bottom and change
string mapPath = "resources\interface\maps"
to
 string mapPath = "resources_override\interface\maps"
Please note that this is not a direct quote, I used backslashes and changed the paths around to what I think should work.
Forward slashes have never worked and I have high doubts that they suddenly do work.

Paths within windows have backslashes
Url's have forward slashes