1. Browse to the central admin site-->Application management
2. Click on Policy for web application in the Application security section
3. Click on Add users. Select the web application and select All zones from the drop-down. Click Next.
4. Type in Nt authority\network service in the Choose users portion and select Full Control and click Finish.
Repeat the steps 3 and 4 for each web application that is present in the farm.
Also make sure that Nt authority\network service account has the same permission in the Central admin site.
(This step needs to be done manually)