In our SharePoint 2013 we use OWA. We can open documents in it, but when we try edit excel workbook (old files and also new) in EWA, it shows box with text:
You can create a copy in a supported format to edit the workbook in the browser. (it is translated by Google Translator)
But in last it was ok. I think, that it isn't same problem like this. I did not find any similar problem yet.