OK, I found *my* problem, it seems quite app-specific, so probably
won't be helpful to others getting this mysterious error code.

Here's what's happening - the app copies a template XLS file with a
new name to a given output location. This new name is determined by
the code, and it appears in some instances it can be " ", resulting in
the template being copied to something like "C:\Location\ .xls".
Which, seems to be a valid windows filename, however, when i try to
open the file with the Excel interop code, it throws the error.