Various operations fail in Plesk: Failed to read data from stream Process output

Symptoms

• When opening Plesk Backup Manager or removing a domain in Plesk, the operation fails with:

  PLESK_INFO: Internal Error: Failed to parse response. Reason: Failed to read data from stream Process output

• When creating a domain in Plesk, the operation fails with:

  PLESK_INFO: Creating physical hosting
  phpinimng failed: Traceback (most recent call last):
  ...
  ImportError: cannot import name 'sysconfig' from 'distutils' (/usr/lib/python3.7/distutils/__init__.py):

• The pmmcli utility fails with "ImportError":

  # /usr/local/psa/admin/bin/pmmcli --help
  ...
  ImportError: No module named libxml2

  ---

  # /usr/local/psa/admin/bin/pmmcli --help
  ...
  ImportError: cannot import name 'etree' from 'lxml' (/usr/lib/python3/dist-packages/lxml/__init__.py)

Cause

/usr/bin/python3 is linked to a non-default Python version.

Resolution

1. Connect to the Plesk server via SSH.

2. Remove the invalid link:

   # unlink /usr/bin/python3

3. Link /usr/bin/python3to /usr/bin/python3.6:

   # ln -s /usr/bin/python3.6 /usr/bin/python3