Error when opening office documents in the browser : Server error - We're sorry. An error has occurred. We've logged the error for the server administrator

Cause :
Apparently this error can be caused by someone installing software updates to Office Web App servers. After you have applied hotfixes you need to recreate the office web app farm.

Troubleshooting:

To start with check to see what the current Office Web App farm settings were :-

Get-OfficeWebAppsFarm

If presented with the error message below, then proceed to RESOLUTION step

It does not appear that this machine is part of an Office Web Apps Server farm

Resolution

New-OfficeWebAppsFarm -Verbose –InternalURL "http://webapp-svr" –AllowHttp -EditingEnabled -OpenFromUrlEnabled