It occurs because the .xml file stores only the required part of the file path. ie, if you move around the file it won't be readable an more.
Two fixes are to 1. move it back to where you saved it and the save as in the res\xml folder or 2. open the .xml with a script editor (I used C++) and rename the filepath for each tileset. Easiest way to do this is to cut and aste from a working map in the same folder.
Remember to make sure submitted maps are saved in the same filepath as the source or it'll make more work for the dev team. Sorry I didn't have the same problem 18 months ago, and I hope this helps eople in the future.
Commander of the Archer's of the Frog
Level: 50, Gold: 65269 WA 1 D 1 BS 1 CS 2 QL 1 IF 3 R 2 MF 1 Level: 25, Gold: 7940 QL 2 IF 2 MF 2