[Pkg-roundcube-maintainers] Bug#649264: 0.5.4+dfsg-1 to 0.6+dfsg-1 package upgrade fails while upgrading the database

[Moonwalker]

Package: roundcube-core
Version: 0.6+dfsg-1

===
-+ Configuring roundcube-core +-
An error occurred while upgrading the database:
/var/lib/dpkg/info/roundcube-core.postinst: 1: /var/lib/dpkg/info/roundcube-core.postinst: cannot create /var/cache/dbconfig-common/backups/roundcube_0.5.4+dfsg-1.mysql: Directory nonexistent
 
Fortunately, /var/cache/dbconfig-common/backups/roundcube_0.5.4+dfsg-1.mysql holds a backup of the database, made just before the upgrade.
===

in /var/log/dpkg.log:
2011-11-19 10:42:27 upgrade roundcube 0.5.4+dfsg-1 0.6+dfsg-1
2011-11-19 10:42:27 status half-configured roundcube 0.5.4+dfsg-1
2011-11-19 10:42:27 status unpacked roundcube 0.5.4+dfsg-1
2011-11-19 10:42:27 status half-installed roundcube 0.5.4+dfsg-1
2011-11-19 10:42:27 status half-installed roundcube 0.5.4+dfsg-1
2011-11-19 10:42:28 status unpacked roundcube 0.6+dfsg-1
2011-11-19 10:42:28 status unpacked roundcube 0.6+dfsg-1
2011-11-19 10:42:28 upgrade roundcube-core 0.5.4+dfsg-1 0.6+dfsg-1
2011-11-19 10:42:28 status half-configured roundcube-core 0.5.4+dfsg-1
2011-11-19 10:42:29 status unpacked roundcube-core 0.5.4+dfsg-1
2011-11-19 10:42:29 status half-installed roundcube-core 0.5.4+dfsg-1
2011-11-19 10:42:30 status half-installed roundcube-core 0.5.4+dfsg-1
2011-11-19 10:42:31 status unpacked roundcube-core 0.6+dfsg-1
2011-11-19 10:42:31 status unpacked roundcube-core 0.6+dfsg-1
...
2011-11-19 10:43:38 configure roundcube-core 0.6+dfsg-1 <none>
2011-11-19 10:43:38 status unpacked roundcube-core 0.6+dfsg-1
2011-11-19 10:43:38 status unpacked roundcube-core 0.6+dfsg-1
2011-11-19 10:43:38 status unpacked roundcube-core 0.6+dfsg-1
2011-11-19 10:43:38 status unpacked roundcube-core 0.6+dfsg-1
2011-11-19 10:43:38 status unpacked roundcube-core 0.6+dfsg-1
2011-11-19 10:43:38 status unpacked roundcube-core 0.6+dfsg-1
2011-11-19 10:43:38 status unpacked roundcube-core 0.6+dfsg-1
2011-11-19 10:43:38 status unpacked roundcube-core 0.6+dfsg-1
2011-11-19 10:43:38 status unpacked roundcube-core 0.6+dfsg-1
2011-11-19 10:43:38 status half-configured roundcube-core 0.6+dfsg-1