Remove PGPASSWORD from the User and System Environment Variables in Windows System Properties.
Reboot the server (hardware reboot) for the change to take effect.
This behavior is related to a known issue with ID 821312 which was corrected in Tableau 2018.2.4 and 2018.3.1.
Note: The error message Did not parse exactly one backup result, got 0 is a generic error message indicating that the backup failed. If the PGPASSWORD or PGUSER environment variables are not set on the computer running Tableau Server, this article does not apply.