First things first, try to start the utility via right-click, 'Run as Administrator' and see if that resolves the issue. With UAC & server 2008, all administrator accounts except for the original 'default' administrator account actually run as a standard user and require elevation. Other options to try are to disable UAC (not recommended), or browse to the folder via the admin network share (\servername\c$\foldername) when setting the output path in the Restore utility.
If I recall correctly, the SBS2011 setup makes you create a secondary/new administrator account as a best-practice thing, so you will probably run into this a lot when dealing with folders, especially with one at the root of C:.
Some more info (written for Win7, but applies):
Start from Windows Vista/Windows 2008, UAC is added which help increase the security level.
If UAC is enabled, all accounts belong to Administrators group (except Administrator account) are actually running as a standard user. They have permission to "run as administrator" when need to access/modify system files and/or registries.
Thus if we only add Administrators in NTFS permission, only Administrator account can access the file without "run as admin". The workaround is to disable UAC (not recommanded) or create a new group with all administrator accounts and add the group to NTFS permission as well.