Assuming the crew sickness occurred prior to departure, this is not (yet) a clear case. But there was at least one recent ruling in the UK where crew sickness was not deemed an extraordinary circumstance:

http://www.bottonline.co.uk/press-re...y-compensation

I would write to Cathay again and ask for compensation (EUR 600) with reference to the above ruling.

You may also want to mention that crew sickness prior to departure was not included in a draft list of extraordinary circumstances by the National Enforcement Bodies:

http://ec.europa.eu/transport/themes...ances-list.pdf

Good luck.