Problem solved, but info posted here to help others.

Using COM calls to Excel (2000, but probably affects all versions), I was occasionally getting the above error (= &H80010001&) returned by OBJRESULT, aparently at random, when I called:
OBJECT GET oExcelApp.WorkBooks.Count TO oVnt

It turned out that this rather unhelpful message actually means "Callee is not ready" and was caused by having a cell in Excel open for editing. Once I pressed Return or Tab in Excel to confirm the edit, the call executed normally.