Server Error in ‘/x’ Application – Excel Web App Error – Solution

Problem:
All of a sudden, Office Web Apps Excel Web App stopped working and gave this Runtime error message on opening Microsoft Excel files from SharePoint 2013:
Server Error in ‘/x’ Application.

Solution: 

This occurs due to the public key which connects to the WOPI application, such as the Office Web Apps Server to the SharePoint farm going out of sync! To resolve the issue, Run from any SharePoint Server:  
Update-SPWOPIProofKey