If it's post-upgrade, it's possible the checkbox to "Reset out of the Box Content" was not selected, in which case the widgets/dashboards would not get "updated" to the most recent version and references metrics that changed names and or were removed. The way you can fix this is by either editing the widget to select the proper metrics, or re-install the Horizon PAK file but this time select both checkboxes.
Please remember that anything modified from out of the box will be RESET so if you have lots of configuration changes to the default dashboards, I would export them so you have copies. As always, take a snapshot prior to any upgrade in case you need to revert!
-K