Comment 4 for bug 1453740

I just encountered this issue when trying to update a trusty server to xenial.
Here's how we "resolved" it.

After do-release-update exited with a similar to the above message, there is a temporary ubuntu-release-upgrader-{random} left over in /tmp.
Enter that directory i.e. cd /tmp/ubuntu-release-upgrader-asd123
edit DistUpgradeCache.py
Head down to def installedTasks(self)
after the line that reads: "for pkg in self:"
add logging.debug("%s \n" % pkg)
You need to make sure this is indented with spaces to the same level as the next line.

save / exit, then run (assuming you are upgrading to xenial)
./xenial

When it crashes this time, you can view /var/log/dist-upgrade/main.log and see which was the last package processed that caused the crash. Remove this package, then proceed with do-release-upgrade as normal. In our case it was a very old wowzaServer package.

This is obviously not a fix, but it got the job done :)
Hope this helps.