Have you any experience with similar problems?
Yes, I've seen this kind of problem, where a URL works fine in all browsers, but not via a programmed HTTP request, returning a server error.
What OS(s) are you working with?
Try the same request with Synapse and FPC's HTTP client. If those work (or don't work), you'll know something more about the problem.
You could also try curl (or nscurl on Mac) to get more data points about the problem.