"Use an external calendar" checkbox setting no longer sticking

On 2 Windows computers the "Use an external calendar" checkbox setting is no longer sticking after installing the latest update to version 6.0.0(37205).  It works when I choose Outlook, but when setting up the next meeting, the checkbox is unselected and I have to select it manually each time.

Is this a bug in this version, or did the update reset some other setting that I need to set to get my "Use an external calendar" checkbox setting to stick?