The call failed on the server; see server log for details
Do you have access to the server logs from my.safaribooksonline.com? If not, then all you know is that your request caused a failure in their system. You could experiment with different requests to see what features of the request cause failures. Or if this happens frequently and unpredictably you could contact them and try to find somebody to discuss the problem with.
By the way your post title is misleading. There is no XML file involved here, the server failed instead of returning one to you.