update-manager crashed with KeyError in __getitem__(): "The cache has no package named 'linux-headers-3.2.0-12'"

Status changed to 'Confirmed' because the bug affects multiple users.

I ran into this issue with a different package by performing the following steps:

1) Launch update-manager
2) Check for updates (There must be updates available)
3) Click on settings
4) Switch to the "Ubuntu Software" tab
5) Choose a different server in "Download from: "
6) Click "Choose server"
7) Authenticate
8) Click "Close"
9) update-manager will now crash as it tries to look up one of the packages that used to be in the cache.

Additionally, the crash won't be reportable because apport thinks it is not an official package as apt-cache policy shows the following:

9:47:51 - flash:[...ch/bug-attachments/plymouth] apt-cache policy update-manager
update-manager:
  Installed: 1:0.156.5
  Candidate: 1:0.156.5
  Version table:
 *** 1:0.156.5 0
        100 /var/lib/dpkg/status

One another machine performing these steps I received a Stacktrace instead of a python Traceback.

1) run update manager.
2) check for updates.
3) shows that no new updates are available, and then crashes.

12.04 upgraded from 11.10

1) Ran update-manager
2) checked for new updates
3) Started downloading updates
4) Crashed and I selected the restart the program option
5) Update finished normally

I have no idea if this is relevant, but my sources.list points to a privately-maintained mirror of the ubuntu repos, not one of the main repositories. I experienced the same bug as others have reported here.

refreshing updates in update-manager first reported connectivity problems, on second try it led to crash. apt-get update && apt-get upgrade worked

Installing updates on clean installed 12.04 after being turned off a coupl of days, running under virtualbox ..

Got this error trying upgrade ubuntu 12.04 beta2 to 12.04