It's been raised in our DXC environment that AB testing is broken and before spending large amounts of time looking doesn't anyone have any ideas?

When the ThumbnailStore URL is called when trying to create an A/B test the following error is shown

ArgumentException: Value cannot be null or empty.
Parameter name: controllerName
Stack trace: ArgumentException: Value cannot be null or empty. Parameter name: controllerName
at System.Web.Mvc.HandleErrorInfo..ctor(Exception exception, String controllerName, String actionName)
at System.Web.Mvc.HandleErrorAttribute.OnException(ExceptionContext filterContext)
at System.Web.Mvc.ControllerActionInvoker.InvokeExceptionFilters(ControllerContext controllerContext, IList`1 filters, Exception exception)
at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>c__DisplayClass21.b__1e(IAsyncResult asyncResult)
at System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeAction(IAsyncResult asyncResult)
at EPiServer.Shell.Services.Rest.RestControllerBase.EndExecute(IAsyncResult asyncResult)
at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
at System.Web.HttpApplication.ExecuteStepImpl(IExecutionStep step)
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

This sounds like something in the routing is not working and it's not registering the controller for the AB. Any ideas?