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Last tested: Jun 11, 2020
When your content validator is showing "unknown field" for a field, but the field still exists in the Look, from 7.8 Looker version, the content validator now shows errors for scheduled plan filters.
The clock icon will indicate that the error is due to a bad filter in the schedule. If you click on the replace/remove button, you will also see that changing the filter will only affect the schedule for that Look. You will have to recreate a new schedule with the new filter set.