Excel version 15 (2013), in Windows 8.
Steps to reproduce:
The Excel object gets broken and yields errors on subsequent COM messages.
If there is an open workbook besides the XLA, everything works fine. But if the XLA is the only open book, it fails.
Question: It is just me, or this is a COM Excel server bug?
It looks like there is a misalignment between the Excel GUI and the Excel COM server introduced in that version.
In older versions both the server and the GUI behaved in the same way when the last workbook was closed: an empty GUI (i.e., with no workbook on it) remained visible, provided it was visible right before closing the last book.
However, this behavior changed in version 15 (or so) and now the GUI becomes hidden when you close the last book, while the server doesn't. This causes that if your COM client closes the last workbook when the GUI is visible, the server won't turn it invisible and thus it will acquire somehow an incompatible state, which is not possible to reach from the GUI.
The problem seems to have been introduced when Excel the GUI changed in that it now assigns an independent window to every workbook.
The solution, therefore, is for the client to make sure it turns the GUI invisible when closing the last open book.
This issue seems to be aggravated if you loaded an Add-in. Maybe what happens is the Add-in (and this is my guess) is associated some hidden window and now the problem the client has to fix involves both, the window of the last book (now closed) and the window associated to the add-in.