Debug: Outlook office addin (apps for office) fails to load, errors out

Recently one of my developer customer reported that he designed a new Outlook addin (apps for office) which fails to load and it throws the error “This app could not be started. Close this dialog to ignore the problem or click "Restart" to try again.”

During troubleshooting we noticed that this issue occur because protected mode is not enabled for the Restricted Sites zone in Internet Explorer. By re-enabling the protected mode for restricted sites in IE, helped them moving forward.

[Update] For additional info, please refer the related blogpost.

Hope this helps.