You cannot use Outlook Web App after you enable the "System cryptography: Use FIPS algorithms for encryption, hashing and signing" security policy on an Exchange Server 2010 server

Exception
Exception type: System.TypeInitializationException
Exception message: The type initializer for 'Microsoft.Exchange.Data.Storage.GccUtils' threw an exception.

Call stack
----------

Microsoft.Exchange.Data.Storage.GccUtils.SetStoreSessionClientIPEndpointsFromHttpRequest(StoreSession session, HttpRequest httpRequest) 
Microsoft.Exchange.Clients.Owa.Core.OwaWindowsIdentity.CreateMailboxSession(ExchangePrincipal exchangePrincipal, CultureInfo cultureInfo, HttpRequest clientRequest) 
Microsoft.Exchange.Clients.Owa.Core.UserContext.CreateMailboxSession(OwaContext owaContext) 
Microsoft.Exchange.Clients.Owa.Core.UserContext.Load(OwaContext owaContext) 
Microsoft.Exchange.Clients.Owa.Core.RequestDispatcher.CreateUserContext(OwaContext owaContext, UserContextKey userContextKey, UserContext& userContext) 
Microsoft.Exchange.Clients.Owa.Core.RequestDispatcher.PrepareRequestWithoutSession(OwaContext owaContext, UserContextCookie userContextCookie) 
Microsoft.Exchange.Clients.Owa.Core.RequestDispatcher.InternalDispatchRequest(OwaContext owaContext) 
Microsoft.Exchange.Clients.Owa.Core.RequestDispatcher.DispatchRequest(OwaContext owaContext) 
Microsoft.Exchange.Clients.Owa.Core.OwaModule.OnPostAuthorizeRequest(Object sender, EventArgs e) 
System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() 
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

Inner Exception:
------------------
Exception type: System.Reflection.TargetInvocationException
Exception message: Exception has been thrown by the target of an invocation.

Call stack:
----------
System.RuntimeMethodHandle._InvokeConstructor(Object[] args, SignatureStruct& signature, IntPtr declaringType) 
System.Reflection.RuntimeConstructorInfo.Invoke(BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture) 
System.Security.Cryptography.CryptoConfig.CreateFromName(String name, Object[] args) 
System.Security.Cryptography.SHA256.Create() 
Microsoft.Exchange.Data.Storage.DatacenterServerAuthentication..ctor() 
Microsoft.Exchange.Data.Storage.GccUtils..cctor()

Inner Exception
Exception type: System.InvalidOperationException
Exception message: This implementation is not part of the Windows Platform FIPS validated cryptographic algorithms.

Call stack

System.Security.Cryptography.SHA256Managed..ctor()

Log Name:      Application
Source:        MSExchange Common
Date:          10/5/2009 4:08:36 PM
Event ID:      4999
Task Category: General
Level:         Error
Keywords:      Classic
User:          N/A
Computer:      Contoso
Description:
Watson report about to be sent for process id: 4092, with parameters: E12IIS, c-RTL-AMD64, 14.00.0639.020, OWA, mscorlib, S.S.C.SHA256Managed..ctor, System.TypeInitializationException, 7d8c, 02.00.50727.4016.
ErrorReportingEnabled: True