SharePoint 2013 web application Backup Script error: Cannot convert value to type System.String

In SharePoint 2013 environment one of the web application backup is failed and rest of all the web applications are successful

The error message was showing like below, 

Backup script error: Cannot convert value to type System.String

I found the problem that is backup running user account does not have access rights to few site collections inside the web applications. 

Solution:

Going to that particular web application and check if all the site collections does have the full access rights to the account which you used to run the backup job, If not grant full rights.

This worked for me ..