I have had exactly the same problem since acquiring my 770 from eBay last week. I upgraded to the latest software version 39-14 on receiving the tablet, and have been unable to install any new applications since - particularly galling since I bought the tablet to make use of mediastreamer. I get "Unable to Download. Package not found" from Application Manager - even with the latest refresh. If I download the packages from Maemo.org and then try to install from file, the main package fails because clinkc-av isn't available, but clinkc-av gives "Incompatible package" - very frustrating.

Chartreuse's explanation seems plausible - does anyone know if a solution is being worked on?

It would give some comfort if there was somewhere to report these things - but as a newbie I haven't been able to find anywhere yet. Is there a forum which is watched by the people who run Tableteer?