Lync Client don´t get the address book and prompts for Credentials!

Hello

We get the following error on the lync server when the clients login and try to download the address book.

Also a popup for credentials apears on the clients!

An unhandled exception was encountered in WebTicketService service.

Exception Details. System.Security.Cryptography.CryptographicException:
Invalid provider type specified.

   at
Microsoft.Rtc.Internal.WebServicesAuthFramework.AuthUtitlities.FindAndValidateTokenTransferCert(String
certSN, String certIssuer)
   at
Microsoft.Rtc.Internal.WebServicesAuthFramework.WebTicketKeyStore.InitSigningCredentials()
  
at
Microsoft.Rtc.Internal.WebServicesAuthFramework.WebTicketKeyStore.GetSigningCredentials()
  
at
Microsoft.Rtc.Internal.WebTicketService.WebTicketService.GenerateSamlToken(OCSIdentity
ocsId, String sipuriRenew, RequestSecurityToken rst, SecurityKeyIdentifier
proofKeyIdentifier, EndpointAddress audienceAddress)
   at
Microsoft.Rtc.Internal.WebTicketService.WebTicketService.DoIssueToken(Message
rstMessage)
   at SyncInvokeIssueToken(Object , Object[] , Object[] )
  
at System.ServiceModel.Dispatcher.SyncMethodInvoker.Invoke(Object instance,
Object[] inputs, Object[]& outputs)
   at
System.ServiceModel.Dispatcher.DispatchOperationRuntime.InvokeBegin(MessageRpc&
rpc)
   at
System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage5(MessageRpc&
rpc)
   at
System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage4(MessageRpc&
rpc)
   at System.ServiceModel.Dispatcher.MessageRpc.Process(Boolean
isOperationContextSet)
Cause: Application error. Please look through the
exception details for more information.
Resolution:
Restart the server. If
the problem persists contact product support.
Event Xml:
<Event
xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
 
<System>
    <Provider Name="LS Web Components Server" />
   
<EventID Qualifiers="50226">4096</EventID>
   
<Level>2</Level>
    <Task>1074</Task>
   
<Keywords>0x80000000000000</Keywords>
    <TimeCreated
SystemTime="2012-09-10T11:16:34.000000000Z" />
   
<EventRecordID>209783</EventRecordID>
    <Channel>Lync
Server</Channel>
   
<Computer>XXX.local</Computer>
    <Security
/>
  </System>
  <EventData>
   
<Data>WebTicketService</Data>
   
<Data>System.Security.Cryptography.CryptographicException: Invalid
provider type specified.

   at
Microsoft.Rtc.Internal.WebServicesAuthFramework.AuthUtitlities.FindAndValidateTokenTransferCert(String
certSN, String certIssuer)
   at
Microsoft.Rtc.Internal.WebServicesAuthFramework.WebTicketKeyStore.InitSigningCredentials()
  
at
Microsoft.Rtc.Internal.WebServicesAuthFramework.WebTicketKeyStore.GetSigningCredentials()
  
at
Microsoft.Rtc.Internal.WebTicketService.WebTicketService.GenerateSamlToken(OCSIdentity
ocsId, String sipuriRenew, RequestSecurityToken rst, SecurityKeyIdentifier
proofKeyIdentifier, EndpointAddress audienceAddress)
   at
Microsoft.Rtc.Internal.WebTicketService.WebTicketService.DoIssueToken(Message
rstMessage)
   at SyncInvokeIssueToken(Object , Object[] , Object[] )
  
at System.ServiceModel.Dispatcher.SyncMethodInvoker.Invoke(Object instance,
Object[] inputs, Object[]&amp; outputs)
   at
System.ServiceModel.Dispatcher.DispatchOperationRuntime.InvokeBegin(MessageRpc&amp;
rpc)
   at
System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage5(MessageRpc&amp;
rpc)
   at
System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage4(MessageRpc&amp;
rpc)
   at System.ServiceModel.Dispatcher.MessageRpc.Process(Boolean
isOperationContextSet)</Data>
 
</EventData>
</Event>

Any sugesstions will be appreciated

/Andreas