Comment 2 for bug 371023

Some more info. The following was the output I saw in my terminal which helped me to fix the server (fix in next comment):

Configuration file `/etc/default/slapd'
 ==> Modified (by you or by a script) since installation.
 ==> Package distributor has shipped an updated version.
   What would you like to do about it ? Your options are:
    Y or I : install the package maintainer's version
    N or O : keep your currently-installed version
      D : show the differences between the versions
      Z : background this process to examine the situation
 The default action is to keep your current version.
*** slapd (Y/I/N/O/D/Z) [default=N] ? n
Installing new version of config file /etc/init.d/slapd ...
  Backing up /etc/ldap/slapd.conf in /var/backups/slapd-2.4.11-0ubuntu6.1... done.
  Moving old database directories to /var/backups:
  - directory o=cejil... done.
  Loading from /var/backups/slapd-2.4.11-0ubuntu6.1:
  - directory o=cejil... failed.

Loading the database from the LDIF dump failed with the following
error while running slapadd:
    ldif_read_file: Not a directory for "/etc/ldap/slapd.conf/cn=config.ldif"
    slapadd: bad configuration directory!
dpkg: error processing slapd (--configure):
 subprocess post-installation script returned error exit status 1
Setting up libclass-accessor-perl (0.31-2) ...
Setting up libio-string-perl (1.08-2) ...
Setting up libtimedate-perl (1.1600-9) ...
Setting up libparse-debianchangelog-perl (1.1.1-2ubuntu1) ...
Setting up ubuntu-minimal (1.140) ...
Setting up grub (0.97-29ubuntu53) ...
Installing new version of config file /etc/kernel/prerm.d/last-good-boot ...

Setting up linux-image-server (2.6.28.11.15) ...
Processing triggers for libc6 ...
ldconfig deferred processing now taking place
Processing triggers for python-support ...
Processing triggers for initramfs-tools ...
update-initramfs: Generating /boot/initrd.img-2.6.28-11-server
Errors were encountered while processing:
 slapd

Could not install the upgrades

The upgrade is now aborted. Your system could be in an unusable
state. A recovery will run now (dpkg --configure -a).

Please report this bug against the 'update-manager' package and
include the files in /var/log/dist-upgrade/ in the bug report.
E:Sub-process /usr/bin/dpkg returned an error code (1)

Setting up slapd (2.4.15-1ubuntu3) ...
  Backing up /etc/ldap/slapd.conf in /var/backups/slapd-2.4.11-0ubuntu6.1... done.
  Moving old database directories to /var/backups:
  Loading from /var/backups/slapd-2.4.11-0ubuntu6.1:
  - directory o=cejil... failed.

Loading the database from the LDIF dump failed with the following
error while running slapadd:
    ldif_read_file: Not a directory for "/etc/ldap/slapd.conf/cn=config.ldif"
    slapadd: bad configuration directory!
dpkg: error processing slapd (--configure):
 subprocess post-installation script returned error exit status 1
Errors were encountered while processing:
 slapd

Upgrade complete

The upgrade has completed but there were errors during the upgrade
process.

Could not install the upgrades

The upgrade is now aborted. Your system could be in an unusable
state. A recovery will run now (dpkg --configure -a).

Please report this bug against the 'update-manager' package and
include the files in /var/log/dist-upgrade/ in the bug report.
installArchives() failed

Setting up slapd (2.4.15-1ubuntu3) ...
  Backing up /etc/ldap/slapd.conf in /var/backups/slapd-2.4.11-0ubuntu6.1... done.
  Moving old database directories to /var/backups:
  Loading from /var/backups/slapd-2.4.11-0ubuntu6.1:
  - directory o=cejil... failed.

Loading the database from the LDIF dump failed with the following
error while running slapadd:
    ldif_read_file: Not a directory for "/etc/ldap/slapd.conf/cn=config.ldif"
    slapadd: bad configuration directory!
dpkg: error processing slapd (--configure):
 subprocess post-installation script returned error exit status 1
Errors were encountered while processing:
 slapd

Upgrade complete

The upgrade has completed but there were errors during the upgrade
process.