If you happen to get this problem when trying to access an excel file to generate lets say HTML from the Excel file content then there are two things that are important to check if you KNOW that you have configured everything else correctly.
1. Check that you account that runs your excel service has sufficient privilages to the content database of the web application that you are using with Excel Services. This could be your Intranet website.
2. Make sure that your file extension is actually .xlsx NOT .xls. This also caused problems when I was wondering WTF(please excuse my language) is going on.