I've been seeing this issue with a few of my users ever since disabling SharePoint Drafts. When trying to open a file, specifically an Excel spreadsheet, they'd get a message that the File Conversion in Process and then another that the Converter Failed to Save File. The strange thing is they weren't trying to covert anything or even to open it in Office Web Apps. If they open the file in Read Only mode the file opens fine and then they can click on the Edit Worksheet button and it works fine. But if they click on the drop-down context menu to Edit in Office Excel or if they click on it and choose the Edit radio button, they get this message.

After much searching and seeing posts to make sure this was installed or reinstall that or change this or modify that - all with no success. I ran across a blog post that was so simple and it actually worked. Much thanks to Amol Ghuge's post for saving the day!