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After the SharePoint administrator has installed, configured, and activated Office Web Apps, end users might receive an error message when they try to view or edit Microsoft Word, Microsoft Excel, Microsoft PowerPoint, or Microsoft OneNote documents in the browser.


These errors might occur for one or more of the following reasons: (Microsoft recommendations)

• The SharePoint administrator has run Office Web Apps Setup, which configures the default open behavior for SharePoint to open Microsoft Word, Excel, PowerPoint, and OneNote documents in the browser by using their associated Web application. However, the SharePoint administrator has not yet started the services, created the service applications, and activated the Office Web Apps feature.
• The Office Web Apps services are not started on the Web front-end server.
• The Office Web Apps service applications and proxies have not been created.
• The Office Web Apps service applications proxies are not associated with the proxy group of the Web application that the site collection belongs to.
• There might be a problem with the permissions for the service account that is used to run the Web application or one of the Office Web Apps services: Excel Calculation Services, PowerPoint Service, or Word Viewing Service. The Web application and service applications must have the appropriate permissions to access the content databases and the configuration database.

Resolution:

The SharePoint administrator can avoid end users' seeing error messages while he or she finishes additional deployment tasks to deploy Office Web Apps on a live production server. We recommended that you enable the OpenInClient feature on existing site collections before you run Setup.

Activate the Office Web Apps feature for the site collection. For information about how to activate the Office Web Apps feature for a site collection.

How to verify that the services has been properly created or not / How to check the status of the services on the servers in the farm:

On the Central Administration Web site, in Application Management, click Manage services on server.

On the Services on Server page, do the following:

1. In the list of services on the selected server, verify that Excel Calculation Services, PowerPoint Service, and Word Viewing Service exist and are started. If a service is not started, click Start.
2. Click the Server button at the top, click Change Server, and then in the Select Server dialog box, click a different server name.
3. Repeat these steps until you have verified that the services exist and are started on all servers in the farm.

Verify that the Office Web Apps service applications and proxies have been created by doing the following:

1. On the Central Administration Web site, in Application Management, click Manage service applications.
2. On the Manage Service Applications page, verify that there the following kinds of service applications exist:
   • Excel Services Application Web service application
   • PowerPoint service application
   • Word Viewing service application

Verify that the Office Web Apps service applications and proxies are associated with the proxy group of the Web application that the site collection belongs to by doing the following:

1. On the Central Administration Web site, in Application Management, click Manage web applications.
2. On the Web Applications Management page, select the Web applications that the site collection belongs to, and then clickService Connections.
3. On the Configure Service Application Associations page, verify that the following kinds of service application proxies are associated with the Web application:
   • Excel Services Application Web service application proxy
   • PowerPoint service application proxy
   • Word Viewing service application proxy

If you have any queries/questions regarding the above mentioned information then please let me know...I would be more than HAPPY to HELP you as well as RESOLVES your issues, Thanks again :-)