Issue: You receive "Sorry, something went wrong" when trying to Open/Preview a document in Office Web Apps.
Cause: This was caused by being logged onto the server as the System Account.
If you collect logs and review them, you will see:
Unhandled exception: System.NotSupportedException: Can not create an identity context for system account user token. at Microsoft.SharePoint.IdentityModel.SPIdentityContext.Create(SPUserToken token
Solution: Make sure you’re not logged in as System Account because you won’t be able to edit or view a document. Log on as a different user and try to access Office Web Apps again.
Configure Office Web Apps for SharePoint 2013