Due to some unknown reasons my Production setup of ERPNext had stopped working.
In a frenzy to get it up and running quickly I backed up using the command bench --site {site} backup. Then I downloaded the backup using SSH.
After that I reinstalled Ubuntu Server and followed the installation process.
After successful installation process I first logged in to the system and then I use the command sudo bench --force --site {site} restore /data_backup.sql.
Then I issued the command bench migrate.
I managed to login and everything seemed to work fine but as soon as I tried to add a PO I got this error message.

In the backup that I downloaded there is file with a long name xxx-xxx-xxx-site_config_backup.json.
This file seems to contain some encryption keys.
Where and how to set/apply these encryption keys?
How to resolve this?