I upgraded to 12.04 LTS and ran Software Center while a few things were running, and crashed. (2 GB RAM on a Netbook.)
On reboot, Ubuntu One interface would not find any programs, neither installed (to run) nor available for download. So I launched Software Center, which opens, but does nothing. I click "Close" button and get a "Force Quit?" box. So I quit.
Did research, learned to run Software Center from Terminal:
2012-04-29 23:14:36,978 - softwarecenter.backend.reviews - WARNING - Could not get usefulness from server, no username in config file
Then tried the below without success:
Reinstallation of software center:
sudo apt-get install --reinstall software-center
Didn't work. Found this in a post: remove the config file for software-center then log out and back in
sudo rm -rf ~/.config/software-center
Didn't work. Reinstalled Software Center with Synaptic Package Manager. Still no dice!
And I am a Linux newbie, so I don't know where the Dickens that config file is. Help appreciated.