This issue seems really odd to me. I first imported the map of the game in the Editor for modifications, following those steps :
Is there an possibility to open the original Titan Quest map so it can be modified?
Here's a link to the game map I extracted:
https://mega.nz/#!94IjWQaa!0tG2WcGzqv70OkDGAZhmtFi8X2cgnHzRCh-1X82DRZE
Extract the "Levels" directory to a mod's Source directory root. Don't use the "Maps" for this. Xpack and Xpack2 are self explanatory.
Here's the World01.wrl and the World01.sd I forgot to add. Place them in the YouMod/Source/Levels/World directory.
https://mega.nz/#!shgmjK7R!zgItf1TSosSLLRCJHQt6Ii-Yfdz-pBWevLrj3582gQg
Once all is in place, right-click the World01.wrl file and "Auto Create Asset". Hit build and wait a while for it to build.
I then modified parts of the map, rebuilt the pathing, and it works! Or, it works in certain places. I edited the map in the act 3 and it works fine. I also edited a part of the map in the Act 4, but my character now can't step in this part of the map at all.
Is there a special manipulation needed for XPack maps?