Receiving run-time error only part of the time
 

When the Sub is called from a workboox event I receive the following
error:
Run-time error '-2147417848(80010108)':
Automation error
The object invoked has disconnected from its clients

However, when I execute the Sub from inside the VB Editor, it
functions correctly. Why?

The triggering event is a ComboBox_Change (without dropdown). and the
Sub routine sets data validation for a named range in the workbook.
The full code can be posted if needed.