Try using the web interface to download a file, ie access https:\\your.tivo.IP.address, with user name "tivo" and password of your media access key. Click on one of the download links and see what error message you get.

I've had recurring problems with the Tivo rejecting transfer requests due to "too many transfers in progress" which, of course, was not really the case as no transfers were in progress. It appears the Tivo is not cleaning up old transfers correctly in some situations.

Powering down, ie pulling the power plug out of the back, and then starting the Tivo seemed to fix it. A restart from the menu did not always seem to work.