Saw this before - first thing is to check and see what happens when you just click on the Excel file - does it open in Excel? (this has to do with default application settings at an OS level).
There are other things that we can check - but I ran into that with a practice that was tight on money so we installed open-source office - works great with the files, and from an OS perspective everything was good.
Down-side, AC is hard-coded and bypasses calling the OS, so that produces problems in that instance.
