In case your code is working correctly on some but not all servers, consider that you are dealing with a slow network bound system. Don't make a lot of children trying to open a bunch of directories at once. Consider it could fail if the server is busy. Check the listing you get back and make sure you aren't trying to get the same (.) directory repeatedly or recursing upwards beyond the public readable folder, etc.

Basically if you can do exactly what your program is supposed to do, perhaps interactively telling it to go one step at a time and seeing what it sees, you should be able to debug.