I ran into an interesting problem the other week with a customer. Windows Update had downloaded and installed an update for Windows 2016, and was now waiting for the servers to be rebooted.
After the servers had been rebooted, we observed that users were not able to connect to any mailboxes homed on those servers, and all the Exchange Virtual Directories were not responding. After checking all the logs, we rolled back the update on one of the servers which didn’t fix the problem. If anything, it intensified the problem by causing the Application Pools to constantly restarting and spiking the CPU to 100%.
Ultimately it was discovered, with Microsoft’s help, that there was a missing file SharedWebConfig.config. This file is normally found in two locations:
The installation of the cumulative update KB4057142 had caused one of the four (two per servers, two servers total) files to go missing; the result being that the server with the missing file stopped working and pulled the other server down with it. Once we copied the file from a different location to replace the missing file, this restored all normal service and the customer was able to complete migrating all the mailboxes to Exchange 2016.