Issue:

After a restore of a Sharepoint 2010 Site Collection is performed you may receive the following error when attempting to run an SSRS report if you are running Reporting Services in Sharepoint Integrated Mode:

The SharePoint site with the id {GUID} could not be found. (rsSPSiteNotFound)

Resolution:

Resolving the issue is a relatively simple process:

1. Browse to the Data Connections on the your Sharepoint Site
2. For each Data Source that is used in any SSRS SP2010 Integrated Report perform the following:
   2.1 Click on the Data Source in the ‘Name’ column and select ‘Edit Data Source Definition’
   2.2 Click on ‘Test Connection’
   2.3 Click on ‘OK’ if the connection test is successful
3. Re-run the SSRS Report and it should work okay now

Is a login box prompt all of a sudden coming up every time you attempt to browse your Sharepoint site, did it previously work okay and works from a different machine with the same user credentials?

If so you could be experiencing a similar problem I have come across with cached credentials.

On Windows 7 browse to:

               Control Panel\User Accounts and Family Safety\Credential Manager

Under the ‘Windows Credentials‘ heading remove the affected users login details that appear for your Sharepoint server.

Try browsing to your Sharepoint site and the login box should not appear.

Thanks to this article for pointing me in the right direction, it also contains other common issues/fixes relating to the login box appearing:
http://sharepointsolutions.blogspot.co.uk/2008/06/how-do-i-make-our-sharepoint-site-stop_17.html?m=0