The panes failing in C02 windows would appear to be quite a common complaint. Contact Velux service department and demand free replacement under 10 years Warranty. Do not be put off by any suggestion that there may be a service charge due to impact damage. Number for service can be found on label on top left hand corner of the opening sash.
Velux helper
November 2009
Get hold of the makers of the velux windows and ask them what could cause this it may be a design fault.
bob
October 2009
I can answer this question...
Why are the interior panes breaking in my Velux Velterm S roof windows?