What does this error mean? Why it appears after years of working?

I have a function that recursively tries to find a control on a page.
For years it has been working well.  Suddenly, I get this error (SabraSoftwareLib is my library of internal functions):
Message: Exception has been thrown by the target of an invocation.
Source: mscorlib
Method: System.Object InvokeMethod(System.Object, System.Object[], System.Signature, Boolean)
Stack Trace:    at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor)
   at System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments)
   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
   at System.Web.UI.WebControls.ObjectDataSourceView.InvokeMethod(ObjectDataSourceMethod method, Boolean disposeInstance, Ob ...