Word WebApp encountered an unexpected error /Excel WebApp unable to process the request

In one of my newly created web application, the users tried to open Excel/Word documents using browsers. They were getting the following error and I had to investigate.

 I checked all the services on the WFE and Word viewer service, Excel calculation service were running. When i checked the event viewer for errors, I found the following in event viewer, which ringed the bell

Then I checked the Central Admin->Application Management-> Select the web application -> Service connections -> Configure service application associations and found that Excel Service and Word viewing service is not configured for this web application. When I enabled those for this application, users were able to view Excel/Word using browser. Hope this helps for you!!!

Excel Web App - An error occured

Users were trying to open the Excel files using Excel Web App in my SharePoint 2010 farm. They were getting "An error occurred Please try again" error dialog.

Also some of the users were not able to see the Indicators of Indicator Details web part. They were getting "An Error Occurred" against each indicator.  I checked all the settings Excel Services and finally found that the "Excel Calculation Service" was stopped. Once it is started, users were able to open excel files and Indicators started showing up. Hope this information is useful for you.