If you have been following my blog, I recently migrated my Exchange 2010 server to a new 2016 server. There have been a few hiccups along the way however for the most part the experience has been good and user disruption minimal, until I encountered Event ID 15021.
Last night during a maintenance window I applied my Microsoft updates and rebooted the server. Shortly after reboot I attempted to open Outlook and it failed with a server unavailable error. OWA and ECP both showed BLANK white screens. I began to panic a little because this is probably the IT guys worst nightmare! Working in the industry for over 20 years I put my panic aside and began my standard troubleshooting procedure.