Unable to open the Plesk Backup Manager: Failed to parse response

Symptoms

1. Unable to access the Plesk Backup Manager:

   PLESK_INFO: Internal Error: Failed to parse response. Reason: Failed to read data from stream Process output
   Message   Failed to parse response. Reason: Failed to read data from stream Process output
   File            PMMConnector.php
   Line           831
   Type          PleskPMMResponseException

2. Execution of the pmmcli utility throws the following error:

   # /usr/local/psa/admin/bin/pmmcli --help
   Traceback (most recent call last):
   File "/opt/psa/admin/sbin/pmmcli", line 5, in <module>
   import pmmcli
   File "/opt/psa/admin/share/pmmcli/pmmcli.py", line 8, in <module>
   import libxml2
   ImportError: No module named libxml2

3. In /var/log/plesk/panel.log the following error can be found:

   CONFIG_TEXT: ERR [panel] PleskPMMResponseException: Failed to parse response. Reason: Failed to read data from stream Process output:
   file: /opt/psa/admin/plib/backup/protocol/PMMConnector.php
   line: 831
   code: 0

4. libxml2 library is installed:

   # dpkg -l | grep libxml2
   ii libxml2:amd64 2.9.1+dfsg1-3ubuntu4.8 amd64 GNOME XML library

5. The /usr/bin/python symlink is different from below:

   # ls -l /usr/bin/python*
   lrwxrwxrwx 1 root root      18 Sep  7 02:02 /usr/bin/python -> /usr/bin/python2.7
   ...

Cause

A custom python version is installed.

Resolution

1. Connect to the Plesk server via SSH.

2. Recreate the symlink to point to /usr/bin/python2.7 with the commands below:

   # unlink /usr/bin/python
   # ln -s /usr/bin/python2.7 /usr/bin/python
   # ls -l /usr/bin/python*
   lrwxrwxrwx 1 root root      18 Sep  7 02:02 /usr/bin/python -> /usr/bin/python2.7
   ...