I have a multi-tenant Umbraco build on v7.3.4. Since last 2 days I have been getting the below error on one of our staging sites. There are multiple installations of the same code but just one site randomly throws the error. The local sites on dev machines and the dev test area are good too. Any help much appreciated

System.InvalidOperationException: An error occurred when trying to create a controller of type 'xxx.Exchange.Web.Controllers.DefaultUmbracoController'. Make sure that the controller has a parameterless public constructor. ---> System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.NullReferenceException: Object reference not set to an instance of an object.
   at Umbraco.Web.UmbracoContext.get_UmbracoUser()