Exchange 2013 Event ID:15021, OWA blank screen

 This is an annoying issue that seems to occur when the SSL certificate is renewed using ECP. Everything appears to be working fine until the server is restarted...

 Under Windows Logs - System there are hundreds of Event ID: 15021 logged with source HttpEvent. Outlook Web Access shows the login page normally but once logged in a blank page is displayed, sometimes with HTTP error 503. Outlook connections and mobile also fail.

The first thing to do is open an elevated cmd prompt and run

netsh http show sslcert

 

At the bottom of the output you should see IP:port 127.0.0.1:443

Copy the Certificate Hash and Application ID into a text document as we will need them in a subsequent step.

In the elevated cmd prompt run:

netsh http delete sslcert ipport=0.0.0.0:444

Next we need to create the command as below substituting the Certificate Hash and Application ID that were previously saved.

netsh http add sslcert ipport=0.0.0.0:444 certhash=CertificateHash appid=ApplicationID

If this command completes successfully you can just reboot the server and the issue should be resolved. If it is not successful check the certificate hash and application ID are in the same format as the screenshot above, no "" around the hash and curly brackets{} around the ID.