I've found a workaround that works for me.
$ rm -rf $HOME/.update-manager-core $ update-manager -d # "upgrade" button should appear
What's more, I can reliably reproduce this bug by doing this: $ rm -rf $HOME/.update-manager-core $ update-manager $ update-manager -d # and now it won't be possible to get the "upgrade" button to appear
I've found a workaround that works for me.
$ rm -rf $HOME/. update- manager- core
$ update-manager -d # "upgrade" button should appear
What's more, I can reliably reproduce this bug by doing this: update- manager- core
$ rm -rf $HOME/.
$ update-manager
$ update-manager -d # and now it won't be possible to get the "upgrade" button to appear