I'll bet that you can open a new workbook and close the new workbook without a problem. When you close a workbook that was started by an outside service, the outside service should do the housekeeping. This way the VBA side and the VFP side will be released.
Any time I have exited a worksheet from Excel in a VFP created session, somewhere along the way to shutting down my computer, I have problems. Sometimes it is in VFP, usually it occurs when I shutdown. I get a message that Excel is running with no referencr in the TaskBar.
I have started to keep the task manager on top, when I am developing with these automation servers.

>I am not using code to close the excel application. I simply click in the upper right hand corner of the workbook to close it. This is what it does not like. It is ok if I click in the upper right hand corner of Excel; Excel closes gracefully. If I click to close the workbook only, that is when I get the illegal operation error.