I've got a bunch of Tivo's, but only the Tivo HD unit has this problem. You go to Tivo Central -> Video on Demand.
It says Please wait, and a few SECONDS later, spits you back out to the Tivo Central.
I don't see ANY option to clear the activation code, etc. I've rebooted the Tivo to no avail.
The Tivo Premier XL I have are just fine.