403 Forbidden errors when calling GetUploadURL for iTunes U API

Question

I am trying to test the uploading of XML to iTunes U using the provided
API. However, I am consistently receiving a 403 Forbidden response
(and resulting exception thrown in Java) when I attempt to get the
URL for upload with the call to /WebObjects/Core.woa/API/GetUploadURL/.
I am able to generate successful login URLs with identical calls to
/WebObjects/Core.woa/API/GetBrowseURL/, which successfully authenticate
me to iTunes U, with the same credential.
Furthermore, following a suggestion found online, I have issued calls to
/WebObjects/Core.woa/Browse/ with my debugging string appended, again
with the same credentials, and that SUCCEEDS, telling me that
"Because the received signature and time were valid, the received identity
and credentials were accepted by iTunes U" so I get no further information.
My system clock has the proper time, etc.
I am also able to retrieve an XML manifest of the contents of my site
by issuing a call to /API/ShowTree/, no problem whatsoever.
Any ideas of what might be the problem? Or perhaps some way I might be
able to do further debugging? Putting the debug string on the
GetUploadURL does nothing, the Forbidden overrides me getting any actual
response at all.
Thanks for any assistance.

Answer 1

Just a sanity check here ...
When you use GetUploadURL, and you want to send a web services XML document rather than an ordinary media file, you must also include the type=XMLControlFile token in your token string.
That is, normally, you create a token string this way:
credentials=your-creds&identity=your-identity&time=12345678&signature=big-long-h ex-string
When you want an upload URL for a web services request, you need to add one more token ...
credentials=your-creds&identity=your-identity&time=12345678&signature=big-long-h ex-string
&type=XMLControlFile