Popup for invalid conditional formatting criteria : appears even when valid (but... Read on...)
As I said, this popup appears quite often in my DB, even when the conditional criteria seems to be valid.

But I think I know why : sometimes , a field appears in a grid, but the condition isn't there in the grid (remember : this is a limitation/feature of the conditional formatting : the field-condition needs to be present as a column in the grid for the conditional format to work)
So, taking the example in the screenshot : the Followup field will be formatted only if the zzSQLNRequest columns/field is present in the grid.