I have an add-in that works just fine when there is a single instance of it
running on a particular machine.

However, if two copies get launched at essentially the same time on the same
machine, I get the following error:

Method 'OnAction' of object '_CommandBarButton' failed.

The runtime error number is 80004005. The failing line of code looks like
this:

Dim cmdButton As CommandBarButton

cmdButton.OnAction = "SomeMacroName"

Again, the assignment statement and the resulting toolbar button work just
fine when a single instance of Excel and the add-in is running ... the error
only happens when two copies are started at the same time.

Any thoughts?

Do multiple copies of Excel share something that's causing this problem?

BTW, am running XL2003.

TIA,

josh