SQL 2005 - Report Manager URL - page cannot be displayed - works fine at SQL Server.

*SQL Server 2005 SP4
*Reporting Services running under NetworkService. Would like to keep this account.
*Users from their clients, cannot access Report Manager URL http://<sql server>/reports does not open
*In Chrome they are prompted for username and password, nothing happens back to username and password
*In IE, browser displays page cannot be displayed.
*On <sql server> directly, the Report Manager URL http://<sql server>/reports works fine - no issues.

Is the service unable to authenticate and check names in Active Directory? Do we need to give the NetworkService access to read active directory, and how is this achieved?

SSRS Log shows

ERROR: Throwing Microsoft.ReportingServices.Diagnostics.Utilities.InternalCatalogException: An internal error occurred on the report server. See the error log for more details., ; 
Info: Microsoft.ReportingServices.Diagnostics.Utilities.InternalCatalogException: An internal error occurred on the report server. See the error log for more details. ---) System.Security.Principal.IdentityNotMappedException: Some or all identity references could not be translated. 
at System.Security.Principal.SecurityIdentifier.Translate(IdentityReferenceCollection sourceSids, Type targetType, Boolean forceSuccess) 
at System.Security.Principal.SecurityIdentifier.Translate(Type targetType) 
at System.Security.Principal.WindowsIdentity.GetName() 
at System.Security.Principal.WindowsIdentity.get_Name() 
at Microsoft.ReportingServices.Diagnostics.UserUtil.GetCurrentWindowsUserName() 
at Microsoft.ReportingServices.Authentication.WindowsAuthentication.GetUserInfo(IIdentity& userIdentity, IntPtr& userId) 
at Microsoft.ReportingServices.WebServer.WebServiceHelper.ConstructRSServiceObjectFromSecurityExtension() 
at Microsoft.ReportingServices.WebServer.Global.ConstructRSServiceFromRequest(String item) 
at Microsoft.ReportingServices.WebServer.Global.get_Service() 
at Microsoft.ReportingServices.WebServer.Global.ShouldRejectAntiDos() 
at Microsoft.ReportingServices.WebServer.Global.Application_AuthenticateRequest(Object sender, EventArgs e)