Similar to this question on the community: https://community.sitecore.net/developers/f/8/t/9690 (but left unsolved) , I've ran into this problem multiple times. Sometimes it seems like it's fixing itself, only to reappear the next day.
Currently this issue is occurring on the BreadcrumbController and the EventListController and I can't seem to get rid of it. I've scanned a lot of pages on the web, including a lot of questions in this community, I double checked my names and namespaces, references and everything, but no help troubleshooting so far.
Error in the preview of the website:
Error Rendering Controller: Breadcrumb. Action: Index: An unhandled exception occurred.
at Sitecore.Mvc.Pipelines.MvcEvents.Exception.ShowAspNetErrorMessage.ShowErrorMessage(ExceptionContext exceptionContext, ExceptionArgs args)
at Sitecore.Mvc.Pipelines.MvcEvents.Exception.ShowAspNetErrorMessage.Process(ExceptionArgs args)
at (Object , Object[] )
at Sitecore.Pipelines.CorePipeline.Run(PipelineArgs args)
at Sitecore.Pipelines.DefaultCorePipelineManager.Run(String pipelineName, PipelineArgs args, String pipelineDomain)
at Sitecore.Mvc.Pipelines.PipelineService.RunPipeline[TArgs](String pipelineName, TArgs args)
at Sitecore.Mvc.Filters.PipelineBasedRequestFilter.OnException(ExceptionContext exceptionContext)
at System.Web.Mvc.ControllerActionInvoker.InvokeExceptionFilters(ControllerContext controllerContext, IList`1 filters, Exception exception)
at System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext controllerContext, String actionName)
at System.Web.Mvc.Controller.ExecuteCore()
at System.Web.Mvc.ControllerBase.Execute(RequestContext requestContext)
at Sitecore.Mvc.Controllers.ControllerRunner.ExecuteController(Controller controller)
at Sitecore.Mvc.Controllers.ControllerRunner.Execute()
at Sitecore.Mvc.Presentation.ControllerRenderer.Render(TextWriter writer)
at Sitecore.Mvc.Pipelines.Response.RenderRendering.ExecuteRenderer.Render(Renderer renderer, TextWriter writer, RenderRenderingArgs args)
Is there something I'm overlooking? Any help is appreciated.