Comment 9 for bug 838787

When upgrading recently, I encountered this bug on multiple systems. My hosts have a sources.list file that points at a local server running approx. This approx server is not a valid mirror. After running do-release-upgrade, I received the missing 'ubuntu-minimal' message. In my case, update-manager commented out all lines in sources.list which pointed to my approx server (since it was not a valid mirror).

I found two possible solutions others may try.

One, it is possible to have update-manager allow third party sources as described in a comment at:

http://bethesignal.org/blog/2009/04/13/approx-package-caching-ubuntu-debian-lovers/

Two, if your sources.list contains both valid and invalid mirrors, update-manager will comment out the invalid mirrors, leaving the valid mirrors. If none of the remaining valid mirrors is the main repository, you get the missing 'ubuntu-minimal' message. If your sources.list ONLY contains invalid mirrors, then update-manager will ask if you want to continue with the invalid mirrors, rewrite your sources.list, and continue with the upgrade. So, if your main repository is an invalid mirror, but you have other lines containing valid mirrors, try commenting out the valid mirrors during the upgrade.