Word Web App cannot open this document for viewing because of an unexpected error (Word Web App no puede abrir este documento por un error inexperado)

I encountered this error on a newly created site collection

Word Web App cannot open this document for viewing because of an unexpected error. To view this document, open it in Microsoft Word.

One look in SharePoint log files with ULS viewer did not reveal much:

w3wp.exe (0x1A74)    0x22F4    Office Web Apps    Office Viewing Architecture    b4vn    Medium    Conversion response received: CantFindSourceDocument    41e96340-f0a6-4b66-b471-83b9d2026d61

 

The problem was that service account which is used for Office Web Apps, in particular Word Viewing Service Application did not have proper permissions (db_owner) on the content database where the site collection is stored.

After setting the permissions, I got another cryptic error which fortunately I encountered before and was an easy fix

Word Web App encountered an unexpected error

To fix this, just close all browser sessions on the client computer and start over again.

Cause of this error was actually end user interaction. User opened Internet Explorer 9 and accessed a site where documents were stored. After that, user opened same site from the same computer using Firefox 7 and Chrome 14, but with different user account. I guess this somehow messed up credentials on the client computer and Office Web Apps couldn’t display office documents. At the same time, users were able to access same document using Office Web Apps from other computers. Simple client computer restart solved the issue.

Fuentes:

http://sharepoint.tejic.com/2012/02/15/how-to-fix-word-web-app-cannot-open-this-document-for-viewing-because-of-an-unexpected-error/