Microsoft Office Add-ins using the Office Add-in platform are designed to run in isolation, using a low-right sandbox. Several features, such as the task pane view, use web views provided using Internet Explorer APIs. To ensure these components execute HTML script in low rights mode, Office explicitly requires the browser object to run in Protected Mode.
Currently, IE does not support this from the API unless Protected Mode is enabled for the Restricted Sites zone in IE itself. This is typically not a problem since this is the default setting, and rarely will users disable it. However, if disabled (by user action or by GPO), these Office add-in types may report an error when starting and refuse to load. It is an expected failure under that condition and is blocked for security reasons.