I tried to one-click install the version of mplayer at garage.maemo.org. The application manager started its usual dance and then blanked. If I tap the menu or the X on the right, nothing happens for maybe 30 seconds, and then a dialog box pops up saying 'Application manager is not responding. Close application?'

I tried rebooting, no help.

I tried dpkg --force-depends --purge osso-application-installer
followed by apt-get install osso-application-installer.

No help.

Does anyone have any ideas? I have a feeling I won't get very far without a working Application Manager!

Turns out there was a long thread on this elsewhere. Bottom line: I changed the hostname of my N800, so I also needed to edit the line for localhost in /etc/hosts to use the new name. A reboot and Application Manager is working again.