Cause 1
You are calling IDispatch::Invoke() with wFlags set only to DISPATCH_METHOD and the dispid identifies a property.
Resolution 1
The Win32 SDK documentation for IDispatch::Invoke() states that if the dispid represents a property, you must also set the DISPATCH_PROPERTYGET flag. This problem commonly occurs when you use Microsoft Visual C++ to generate class wrappers from the Microsoft Office 95 type libraries. You have to explicitly modify the generated .cpp file for any InvokeHelper() call that retrieves a property to use the DISPATCH_PROPERTGET flag. If you use the Microsoft Office 97 type libraries, this is done automatically by the Microsoft Visual C++ Class Wizard.
Cause 2
You are using the wrong IDispatch pointer.
Resolution 2
This typically occurs when you use OLE Automation with the Microsoft Office 97 applications, or any application that has a hierarchical object model. You may have initialized a COleDispatchDriver class, representing an object X, with the IDispatch pointer for an object Y. This error may or may not occur with Microsoft Office 95, or other applications with only one IDispatch implementation. This is because object X and object Y could both be implemented through one IDispatch interface, and thus no conflict will occur.
Cause 3
You are using an invalid dispid.
Resolution 3
The server does not recognize the dispid you have specified in the call to IDispatch::Invoke(). Verify that this is the correct dispid, possibly through IDispatch::GetIDsOfNames().