Comment 2 for bug 1624836

apport checks for a bug pattern before filing a bug report, in the case of the bug which you were reporting it matched the bug pattern for bug 996916. That pattern looks like:

    <pattern url="https://launchpad.net/bugs/996916">
        <re key="Package">^(ubuntu-release-upgrader-core |update-manager)</re>
        <re key="VarLogDistupgradeMainlog">blacklist expr '\^postgresql-.*\[0-9\]\\.\[0-9\].*' matches</re>
    </pattern>

From the bugs I'd seen before most of them failed to calculate the upgrade, however in your log file (Mainlog.txt) the message appears during the obsolete package check.

2016-09-18 12:53:10,503 DEBUG Start checking for obsolete pkgs
2016-09-18 12:53:18,126 DEBUG blacklist expr 'update-manager-core' matches 'update-manager-core'
2016-09-18 12:53:18,127 DEBUG skipping 'update-manager-core' (in removalBlacklist)
2016-09-18 12:53:18,127 DEBUG 'update-manager-core' scheduled for remove but not safe to remove, skipping
2016-09-18 12:53:23,737 DEBUG blacklist expr 'update-manager-core' matches 'update-manager-core'
2016-09-18 12:53:23,737 DEBUG package 'python3-distupgrade' has unwanted removals, skipping
2016-09-18 12:53:25,652 DEBUG 'python3-distupgrade' scheduled for remove but not safe to remove, skipping
2016-09-18 12:53:25,980 DEBUG blacklist expr '^postgresql-.*[0-9]\.[0-9].*' matches 'postgresql-9.3'
2016-09-18 12:53:25,980 DEBUG skipping 'postgresql-9.3' (in removalBlacklist)
2016-09-18 12:53:25,980 DEBUG 'postgresql-9.3' scheduled for remove but not safe to remove, skipping

The bug pattern should be modified so that it only matches bugs where the message appears before upgrading.