After upgrading Debian 6 to 7 I wanted to upgrade openSSL:

apt-get install openssl

This gave me the following error:

doveconf: Fatal: Error in configuration file /etc/dovecot/dovecot.conf

Dovecot must have failed during the upgrade process.

To fix open the dovecot.conf file and update the following line:

mail_plugin_dir = /usr/lib/dovecot/modules/lda

to:

mail_plugin_dir = /usr/lib/dovecot/modules

Dovecot will now be installed properly when you try upgrading any new package.

*BACKUP ALL DATA BEFORE UPGRADING YOUR SYSTEM

Update your sources.list:

vim /etc/apt/sources.list

Add the following sources:

deb http://mirrors.kernel.org/debian/ wheezy main
deb-src http://mirrors.kernel.org/debian/ wheezy main

deb http://security.debian.org/ wheezy/updates main
deb-src http://security.debian.org/ wheezy/updates main

# wheezy-updates, previously known as 'volatile'
deb http://mirrors.kernel.org/debian/ wheezy-updates main
deb-src http://mirrors.kernel.org/debian/ wheezy-updates main

Update the package list:

apt-get update

Update the minimal base system:

apt-get upgrade

Upgrade the rest of the system:

apt-get dist-upgrade

Reboot the system:

reboot

Once the server comes back to life verify you are now running Debian Wheezy:

cat /etc/issue

Sample Output:

Debian GNU/Linux 7 n l

TROUBLESHOOTING

DOVECONF: FATAL: ERROR IN CONFIGURATION FILE /ETC/DOVECOT/DOVECOT.CONF