I had this same problem for over a month, and finally I managed to reflash my N800.
But .. app. manager is still acting flakey, a first it wouldn't install my backup apps, then it wouldn't install them all. Everytime I run it, it fails to update, and if I install an app. AM then falls over with an error.

No idea what to do, as I reflashed with the newest os2008 yesterday from Nokia.

BT