Da quando in poi non si traduce "serio" con "serious"? You know the Guardian is a highly respected and authoritative newspaper, but as a well known poster here recently pointed out sometimes it is just so inaccurate that it can hardly be taken seriously at all!
http://www.proz.com/kudoz/italian_to_english/law_general/450...In English, as in Italian "not serious" can mean any of a number of things, where "frivolous or light hearted" is just one of many. Examples include, not warranting attentive consideration (the meaning here), not warranting a prison sentence, not warranting hospital treatment, etc. Although for some of these "grave" would be more usual in Italian.