At a guess you need to URLencode the URL. It's probably the @ sign in the URL which is confusing things. IE will do the encoding automatically which would explain why it worked in the browser.
I've tried this myself on windows using curl, and not had any problems, I did have to add the -k option to curl to get around curl not working with our ssl cert.
I did not have to do any url encoding, and the only way I could reproduce your problem was by actually using invalid credentials.
If u want to PM, feel free todo so. I am the head of the dev team at buy.at, and also the person in charge of product feeds, so hopefully I know enough to help sort out your probems.