After going through loads of forums and blogs with people having the same or similar issues.
I tried all the recommended solutions but none of it worked for me.
The only time I did not receive this issue was when I logged into IE with the Admin User.
But this solution was not going to work for everyone.
As a solution / workaround I had to configure IIS,
- the application pool identity being the Admin User
- Update the Basic setting of the site to use "Application user (pass-through authentication)" as the Path Credentials
- Open IIS
- Click on Application pools
- Right click on the application pool and select "Advanced Setting".
- Then select Identity.
- Select "Custom account" and set it to the Admin user.
- Select Ok.
- Update the Basic setting of the site. Select the site.
- Click "Basic Settings".
- Click "Connect as..."
- Select "Application user (pass-through authentication)"