Cannot delete attribute(binding) from the portal schema

I'm currently seeing the following error when I try to delete an attribute binding (after which I want to delete the attribute).

Delete "attributeBindingName" failed

If I go to the requests I see the following as matched MPRs: Administration - Schema: Administrators can delete non-system schema related resources

And ofcourse it says "failed"

In the FIM Service Tracelog:

Microsoft.ResourceManagement.WebServices.Exceptions.UnwillingToPerformException: Other ---> Procedure: ReRaiseException.  Line number: 37.  Message: No resource with this identifier exists: 141319.
   --- End of inner exception stack trace ---
Request '58caa0e0-24b1-48f7-873f-7e0aeae0313a' failed while trying to commit the changes to the database.  Exception: 'Other'.

WS: Delete: UnwillingToPerform

Requestor: urn:uuid:7fb2b853-24f0-4498-9534-4e10589723c4
Microsoft.ResourceManagement: Microsoft.ResourceManagement.WebServices.Exceptions.UnwillingToPerformException: Other ---> Procedure: ReRaiseException.  Line number: 37.  Message: No resource with this identifier exists: 141319.
   --- End of inner exception stack trace ---
   at Microsoft.ResourceManagement.Utilities.ExceptionManager.ThrowException(Exception exception)
   at Microsoft.ResourceManagement.Data.Exception.DataAccessExceptionManager.ThrowException(SqlException innerException)
   at Microsoft.ResourceManagement.Data.DataAccess.ProcessRequest(RequestType request)
   at Microsoft.ResourceManagement.ActionProcessor.SchemaActionProcessor.ProcessInputRequest(RequestType request)
   at Microsoft.ResourceManagement.ActionProcessor.ActionDispatcher.ProcessInputRequest(RequestType request)
   at Microsoft.ResourceManagement.WebServices.RequestDispatcher.ExecuteAction(RequestType request)
   at Microsoft.ResourceManagement.WebServices.RequestDispatcher.ExecuteAction[ResponseBodyType](RequestType request)
   at Microsoft.ResourceManagement.WebServices.RequestDispatcher.DispatchRequest[ResponseBodyType](RequestType request, Guid requestIdentifier, Object redispatchSingleInstanceKey, Boolean isRedispatch)
   at Microsoft.ResourceManagement.WebServices.RequestDispatcher.DispatchRequest[ResponseBodyType](RequestType request)
   at Microsoft.ResourceManagement.WebServices.RequestDispatcher.DispatchRequest(RequestType request)
   at Microsoft.ResourceManagement.WebServices.ResourceManagementService.Delete(Message request)

I did an exportPolicy.ps1 and searched it. The only place I found that attribute is on users. It's not used in the definition of Sets, MPRs, SRs, ... I also unchecked it on the FIM MA and remove it's attribute flows.

Can an attribute be populated & linked to users when you try to delete it from the Portal?