Attaché: Error "Could not start Attaché desktop. Object reference not set to an instance of an object" after upgrading

If you receive this error after performing an upgrade to Attaché, it will mean there was an issue installing Attaché and you should re-do the upgrade process. See the Attaché Installation and Upgrade Guide.
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In addition, you should make sure that all your Attaché antivirus exclusions are in place. See: Files to exclude in antivirus software.