Problem Description: Forms Based Authenticated users cannot open Microsoft Office documents
You have configured your Web Application to use Forms Based Authentication. Users cannot open Microsoft Office documents such as Microsoft Word or Excel files. Non-Office documents open without problem. When a user tries to open a Microsoft Office document, the Microsoft Office program is launched (such as Word), but the file, stored in a SharePoint document library, is not opened. In stead, the user sees something which is similar to the login page to which a user is redirected for a Web Application configured with Forms as Authentication Type. The document opened is also indicated as being the login.aspx page.