Update Manager got stuck (but not frozen) while installing downloaded updates. What should I do?

Ideally you should skip configuring flashplugin-installer and reconfigure it when you have a stable connection.

You can kill the download processs to proceed without configuring the package, mostly it is wget, or sometimes curl, but here it seems that it downloads via debconf.

To abort the transaction, you can kill dpkg,

sudo killall dpkg

Then remove the lock,

sudo rm /var/cache/apt/archives/lock
sudo rm /var/lib/dpkg/lock

Then when you have a stable connection, execute,

sudo apt-get -f install
sudo dpkg --configure -a

Here's what I did.

First I found the stuck process (number 19123 in my case):

> pstree -p
├─gksu(7266)─┬─precise(9756)
│            ├─precise(9757)─┬dpkg(24158)─update-notifier(19121)─package-data-do(19123)
│            │               └{precise}(9759)

Then I helped out with bug report: https://bugs.launchpad.net/ubuntu/+source/flashplugin-nonfree/+bug/1243090 . By running "sudo gdb" then "attach 19123" and "bt" to create a backtrace. The backtrace helps the volunteers determine what's wrong.

Finally I used "sudo kill 19123" and received error message "Could not install 'update-notifier-common'" "subprocess installed post-installation script returned error exit status 143".

The rest of the install then completed without incident.