DataFormWebPart - Could not retrieve a valid windows identity for username

Hello,

I recently upgraded from SharePoint 2010 -> 2013.

Migrated successfully to Claims-based authentication thanks to this post.

The problem is that I have a web part that is displaying information from a list. The webpart now is broken and I can see the following in the logs:

	94	02/07/2013 10:32:36.76   w3wp.exe (0x3CA0)                         0x1F10  SharePoint Foundation           Claims Authentication           bz7l  Medium    SPSecurityContext: Could not retrieve a valid windows identity for username 'DOMAIN\levente.rog' with UPN 'Levente.Rog@domain.local'. UPN is required when Kerberos constrained delegation is used. Exception: System.ServiceModel.FaultException`1[System.ServiceModel.ExceptionDetail]: WTS0003: The caller is not authorized to access the service. (Fault Detail is equal to An ExceptionDetail, likely created by IncludeExceptionDetailInFaults=true, whose value is: System.UnauthorizedAccessException: WTS0003: The caller is not authorized to access the service.    at Microsoft.IdentityModel.WindowsTokenService.CallerSecurity.CheckCaller(WindowsIdentity callerIdentity)     at Microsoft.IdentityModel.WindowsTokenService.S4UServiceContract.PerformLogon(Func`1 logonOperation, Int32 pid)     at SyncInvo...  5c5ffc9b-f08c-f077-b706-583c7d9d7147
	95	02/07/2013 10:32:36.76*  w3wp.exe (0x3CA0)                         0x1F10  SharePoint Foundation           Claims Authentication           bz7l  Medium    ...keUpnLogon(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.ProcessMessage31(MessageRpc& rpc)     at System.ServiceModel.Dispatcher.MessageRpc.Process(Boolean isOperationContextSet))..  5c5ffc9b-f08c-f077-b706-583c7d9d7147
	96	02/07/2013 10:32:36.76   w3wp.exe (0x3CA0)                         0x1F10  SharePoint Foundation           Claims Authentication           g220  Unexpected  No windows identity for domain\levente.rog.  5c5ffc9b-f08c-f077-b706-583c7d9d7147
	97	02/07/2013 10:32:36.78   w3wp.exe (0x3CA0)                         0x1F10  SharePoint Foundation           Claims Authentication           bz7l  Medium    SPSecurityContext: Could not retrieve a valid windows identity for username 'DOMAIN\levente.rog' with UPN 'Levente.Rog@domain.local'. UPN is required when Kerberos constrained delegation is used. Exception: System.ServiceModel.FaultException`1[System.ServiceModel.ExceptionDetail]: WTS0003: The caller is not authorized to access the service. (Fault Detail is equal to An ExceptionDetail, likely created by IncludeExceptionDetailInFaults=true, whose value is: System.UnauthorizedAccessException: WTS0003: The caller is not authorized to access the service.    at Microsoft.IdentityModel.WindowsTokenService.CallerSecurity.CheckCaller(WindowsIdentity callerIdentity)     at Microsoft.IdentityModel.WindowsTokenService.S4UServiceContract.PerformLogon(Func`1 logonOperation, Int32 pid)     at SyncInvo...  5c5ffc9b-f08c-f077-b706-583c7d9d7147

Any suggestions would be much appreciated.