I Think i see the problem, a blank XML File wont work, it needs a basic structure.
in the webassist/_promaps_cache/_promaps_geocache.xml, add the following code:
<?xml version="1.0" encoding="UTF-8"?>
also, make sure it has write access.
You may nee to run the page a couple times for it to cache all the addresses, if there is still a problem, i will give a call tomorrow.