1) What browser (and version of the browser) are you using? IE7, IE8, FF
2) Do you have Flash 9 or 10 installed? 9
3) What is the actual link that you are using that is failing for you? I have tried to edit a skin using both and I get the same error as noted in the original post.
After I tried the upload2.aspx site I copied the url and pasted it into a new tab and deleted the 2 out of the address so it would use the other but it still fails.
I have also been unable to upload anything over 28mb at either link for over 3 months. I have been getting the exact same error as in the original post for quite some time.