Recently, I was getting this error in the logs:

RunWorkflow: Microsoft.SharePoint.SPException: <Error><CompilerError Line=”0″ Column=”10″ Text=”The type or namespace name 'Nivlag' could not be found (are you missing a using directive or an assembly reference?)” /></Error>     at Microsoft.SharePoint.Workflow.SPNoCodeXomlCompiler.LoadXomlAssembly(String assmNameIn, SPWeb web)     at Microsoft.SharePoint.Workflow.SPWinOeHostServices.CreateInstance(Guid trackingId, SPWorkflow workflow)     at Microsoft.SharePoint.Workflow.SPWinOeEngine.RunWorkflow(Guid trackingId, SPWorkflowHostService host, SPWorkflow workflow, Collection`1 events, TimeSpan timeOut)     at Microsoft.SharePoint.Workflow.SPWorkflowManager.RunWorkflowElev(SPWorkflow originalWorkflow, SPWorkflow workflow, Collection`1 events, SPRunWorkflowOptions runOptions)     

combined with a workflow that would always get “Failed on Start (retrying)” although it never went anywhere past that. 

 After much much research, it turns out that the SharePoint Designer Workflow Extensions which allows for string manipulation functions to be built into workflows, was causing the problem. I wonder now if it had been installed correctly. I'm pretty sure it was, but maybe I missed something. Either way, uninstallation of the assembly, removal of the .actions file, and removing the workflow actions (i was using Replace() ) solved the problem.

I would like to investigate this further since they are really useful and easy to use. Hopefully this will help someone else if they run into this same problem.