After updating Plesk to 18.0.52, Slave DNS Manager cannot be installed and does not sync DNS zones to slave DNS servers: Invalid json data input: ''

Symptoms

• After updating Plesk to 18.0.52, the extension Slave DNS Manager cannot be installed:

  Could not install extension Slave DNS Manager:
  The execution of slave-dns.php has failed with the following message:
  Execution server_dns has failed with exit code 1, stdout: , stderr: ERR [panel] Custom DNS backend has finished with a non-zero error code 1, stderr: ERR [extension/slave-dns-manager] The execution of slave-dns.php has failed with the following message:
  Invalid json data input:
  The execution of slave-dns.php has failed with the following message:
  Invalid json data input:
  Unable to enable Custom DNS backend: invalid DNS backend script.

• Syncing DNS zones to slave DNS servers finishes successfully but in fact is not done, and the same error as above is found in the log file /var/log/plesk/panel.log.

Cause

Plesk bug PPPM-13974.

Resolution

1. Connect to the server using SSH.

2. Download the patch:

   # wget https://support.plesk.com/hc/en-us/article_attachments/14192278946967/Custom.php

3. Back up the original file and replace it with the patched one downloaded in previous step:

   # mv /usr/local/psa/admin/plib/Service/Dns/Connector/Custom.php{,.orig}
   # mv Custom.php /usr/local/psa/admin/plib/Service/Dns/Connector/Custom.php && chown root:root /usr/local/psa/admin/plib/Service/Dns/Connector/Custom.php