App Store doesn't fetch updates anymore

I'm on a MacBook Pro, mid 2009, OS X 10.10.

An update to Yosemite (10.10.2) came out five days ago, but going to the Updates tab in App Store only returns a dialog box with the following message:

An error has occurred
Can't connect to the Apple Software Update server.

I have turned off Firewall, logged out/in from App Store, removed App Store cookies - but despite this and after googling over an hour I'm still clueless.

Any ideas what could be the problem?