ubuntu 64-bit 12.04 upgrading couchbase server from 1.8.1 to 2.0 works but prints out error in the end when starting the server ( * Failed to start couchbase-server dpkg: error processing couchbase-server (--install) )

ubuntu@ip-10-190-47-53:~$ sudo dpkg -i couchbase-server-community_x86_64_2.0.0-1969-rel.deb
(Reading database ... 31702 files and directories currently installed.)
Preparing to replace couchbase-server 1.8.1 (using couchbase-server-community_x86_64_2.0.0-1969-rel.deb) ...

=INFO REPORT==== 30-Nov-2012::22:42:06 ===
Initiated server shutdown** at node ns_1@127.0.0.1 **

=INFO REPORT==== 30-Nov-2012::22:42:14 ===
Stopped ns_server application** at node ns_1@127.0.0.1 **
 * Stopped couchbase-server
libssl0.9.8 is installed. Continue installing
Upgrading previous couchbase ... (1.8.1)
Stopping previous couchbase ... (1.8.1)
 * Failed to stop couchbase-server
Saving previous couchbase config.dat ...
Cleaning symlinks ...
Unpacking replacement couchbase-server ...
Setting up couchbase-server (2.0.0) ...
Upgrading couchbase-server ...
  /opt/couchbase/bin/install/cbupgrade -c /opt/couchbase/var/lib/couchbase/config -a yes
Automatic mode: running without interactive questions or confirmations.
Upgrading your Couchbase Server to 2.0.0-1969-rel.
The upgrade process might take awhile.
Analysing...
Previous config.dat file is /opt/couchbase/var/lib/couchbase/config/config.dat.debsave
Target node: ns_1@127.0.0.1

Couchbase should not be running.
  Please use: /etc/init.d/couchbase-server stop

Database dir: /opt/couchbase/var/lib/couchbase/data

Buckets to upgrade: default

Checking disk space available for buckets in directory:
  /opt/couchbase/var/lib/couchbase/data
  Free disk bucket space wanted: 997652480.0
  Free disk bucket space available: 6136127488
  Free disk space factor: 2.0
  Ok.

Analysis complete.

Copying /opt/couchbase/var/lib/couchbase/config/config.dat.debsave
    cp /opt/couchbase/var/lib/couchbase/config/config.dat.debsave /opt/couchbase/bin/install/../../var/lib/couchbase/config/config.dat
Ensuring bucket data directories.
Ensuring bucket data directory: /opt/couchbase/var/lib/couchbase/data/default
    mkdir -p /opt/couchbase/var/lib/couchbase/data/default
Ensuring dbdir owner/group: /opt/couchbase/var/lib/couchbase/data
    chown -R couchbase:couchbase /opt/couchbase/var/lib/couchbase/data
Ensuring dbdir owner/group: /opt/couchbase/var/lib/couchbase/data
    chown -R couchbase:couchbase /opt/couchbase/var/lib/couchbase/data
Upgrading buckets.
Upgrading bucket: default
    /opt/couchbase/bin/install/../cbtransfer /opt/couchbase/var/lib/couchbase/data/default-data/default couchstore-files:///opt/couchbase/var/lib/couchbase/data -b default --source-vbucket-state=active --destination-vbucket-state=active
cbdbupgrade pid: 3328
  [####################] 100.0% (219036/219036 msgs)
bucket: default, msgs transferred...
       : total | last | per sec
 batch : 2019 | 2019 | 27.5
 byte : 449104863 | 449104863 | 6106649.5
 msg : 219036 | 219036 | 2978.3
done
cbdbupgrade err: 0
Upgrading bucket: default
    /opt/couchbase/bin/install/../cbtransfer /opt/couchbase/var/lib/couchbase/data/default-data/default couchstore-files:///opt/couchbase/var/lib/couchbase/data -b default --source-vbucket-state=replica --destination-vbucket-state=replica
cbdbupgrade pid: 3337
..............................................................................................................................................................................................................
bucket: default, msgs transferred...
       : total | last | per sec
 batch : 1025 | 1025 | 464.5
 byte : 0 | 0 | 0.0
 msg : 0 | 0 | 0.0
done
cbdbupgrade err: 0
Upgrading bucket: default
    /opt/couchbase/bin/install/../cbtransfer /opt/couchbase/var/lib/couchbase/data/default-data/default couchstore-files:///opt/couchbase/var/lib/couchbase/data -b default --source-vbucket-state=pending --destination-vbucket-state=pending
cbdbupgrade pid: 3346
..............................................................................................................................................................................................................
bucket: default, msgs transferred...
       : total | last | per sec
 batch : 1025 | 1025 | 420.4
 byte : 0 | 0 | 0.0
 msg : 0 | 0 | 0.0
done
cbdbupgrade err: 0
Upgrading bucket: default
    /opt/couchbase/bin/install/../cbtransfer /opt/couchbase/var/lib/couchbase/data/default-data/default couchstore-files:///opt/couchbase/var/lib/couchbase/data -b default --source-vbucket-state=dead --destination-vbucket-state=dead
cbdbupgrade pid: 3355
..............................................................................................................................................................................................................
bucket: default, msgs transferred...
       : total | last | per sec
 batch : 1025 | 1025 | 473.6
 byte : 0 | 0 | 0.0
 msg : 0 | 0 | 0.0
done
cbdbupgrade err: 0
Ensuring bucket owner/group: /opt/couchbase/var/lib/couchbase/data/default
    chown -R couchbase:couchbase /opt/couchbase/var/lib/couchbase/data/default

Done.
 * Failed to start couchbase-server
dpkg: error processing couchbase-server (--install):
 subprocess installed post-installation script returned error exit status 1
Processing triggers for ureadahead ...
Errors were encountered while processing:
 couchbase-server
ubuntu@ip-10-190-47-53:~$ service couchbase-server start
 * Must run as root
ubuntu@ip-10-190-47-53:~$ sudo service couchbase-server start
 * couchbase-server is already started
ubuntu@ip-10-190-47-53:~$