I got this error message when I tried to view my workbook via server.
I can see the thumbnails fine, but when I clicked it, it gave me error.
I recently upgraded my desktop version to 5.0.4
Database error 0x80040154: Class not registered
Unable to connect to the Microsoft Excel file "D:\Tableau Server\data\tabsvc\temp\4F3.tmp\Data\CPS_2009\CPS_megaFile_2009_1012.xlsx". Check that you have access privileges for the requested file and that it is not open in another application.
This is because the server does not have microsoft's access 2007 database engine (used to view excel 2007 files) installed. Please go to http://www.tableausoftware.com/community/support/drivers, then download and install the 'Office 2007 Data Connectivity Components' on your server.