Ok, thanks for the details. I was able to go to the feeds section and get the same error. When I did this the top of the feeds view said Feeds - my domain name and below it was a / and a text field for me to enter the name of the feed. I made sure to upload the sitemap file then I entered the name and when I clicked on 'add fee' I got the same error.
I then browsed to a different part of the Site Explorer and then browsed back to the feeds area. When I got back to it I noticed that to the left of the text box you enter the name of the sitemap in I had my domain listed. This is different from the first time I saw this where there was only a slash there. When I tried submitting the site map in the updated interface it went through successfully. So it seems that there might be a problem on the Yahoo Site Explorer when submitting feeds. This was my experience at least.