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I've voted to close this as too localized because the problem isn't in the question at all. It's external to the data provided and specific to your configuration.
Solved. the problem was raised by a trigger enabled on the table.
the problem is that the trigger contain exactly the same code to update the GEOG field when GEO_LAT and LON field are updated... really strange but for now the problem is solved also without 0.0 and 90.0
SELECT geography::STPointFromText('POINT(0 90)', 4326)
doesn't raise any errors for me.