A first chance exception of type System.IO.FileNotFoundException is thrown by the IsolatedStoragetProvider when calling its SaveToStorage() method. Note: the exception does not prevent the manager from saving the UI. To see the error you will need to enable the CLR exceptions from the Visual Studio's Debug-->Exceptions menu Available in LIB version: 2015.1.1805
Well, if you don't have CLR exceptions enabled you can see it as a first chance exception of type 'System.IO.FileNotFoundException' in mscorlib.dll. It pollutes the output when the persistence is saved.