OWA status code 500 and ECP Could not load file or assembly after installing Security Update For Exchange Server 2016 CU7 (KB4045655)

By Mariette Knap exchange security update, http error 500, Could not load file or assembly

After you installed Security Update For Exchange Server 2016 CU7 (KB4045655) you are no longer able to use OWA or ECP on your Exchange Server 2016.

When you try OWA on your Exchange Server 2016 you get “Something went wrong. Your request couldn't be completed. HTTP Status code: 500.”

OWA status code 500 and ECP Could not load file or assembly after installing Security Update For Exchange Server 2016 CU7 (KB4045655)

And you get ‘Could not load file or assembly 'Microsoft.Exchange.Common, Version=15.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The system cannot find the file specified.

The ‘reason’ that this happened is that the security patch was not completely installed because it was not ran from an elevated command prompt and here is how you do that

Start an elevated command prompt
Type Exchange2016-KB4045655-x64-en.msp and let it run. It will take some time to complete. After it is done reboot the server.
```
Exchange2016-KB4045655-x64-en.msp
```