This is what appears on the Clarity Task Log Dialog.
Checking...10:26:41 AM Evaluating 51 tasks... Evaluating Task: Task: 140136: TaskSequence: New Plugin Options: Same mutex is needed, just reuse: {96188ffc-0348-49a2-94c6-8d854786f502}PDFREGISTRYCLARITYPDF An exception while idling was encountered: ArgumentNullException: Value cannot be null. Parameter name: source Stack: at System.Linq.Enumerable.Select[TSource,TResult](IEnumerable`1 source, Func`2 selector) at Clarity.IProjectMonitor.ProjectFileAccessManager.GetProjectAccessorsAsString() at CohortRevitTaskServer.Controller.Process() Too many consecutive exceptions... pausing callbacks...
the issues is the lasted update 20.1.7.0 dropped an extra DLL in place for non-ProjectWise customers.
The fix is to search and delete Clarity.ProjectWiseFileAccessor.dll in your Task Server folders for each version. example C:\Program Files\IMAGINiT Technologies\Clarity Task Server for Revit\2017 , 2018, 2019