Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
Apple releases OS X security update to fight again fake security certificates –
Apple has just released Security Update 2011-005 for OS X. This is a minor update that combats an issue related to fraudulent security certificates issued by DigiNotar.
Mac OS X Server 10.6, Mac OS X 10.4, Mac OS X Server 10.4, Mac OS X 10.6, Mac OS X Server 10.5, Mac OS X 10.5, OS X Lion, Product Security, AirPort, Apple TV, iPhone, iPhoto, iPod touch, iTunes, QuickTime 7, Safari
Security Update 2011-005 is recommended for all users and improves the security of Mac OS X.
For information on the security content of this update, please visit this website:http://support.apple.com/kb/HT1222.
Direct links to the update can be found below: