I'm sorry, I have no idea why you are getting that message. The same thing is happening to me. I downloaded the update yesterday morning. Everything went fine, then when I try to open the Store I get the message in the photo below. I also tried: Tools> Options> Advanced> Unchecked "Check for server Certificate Revocation". Still no deal
I have restarted my computer numerous times and simply do not know what to do at this point.
Just to add: I have also turned off all security on my computer, and during the iTunes update I also updated QuickTime. I did all this from the link on the Apple website. The update for both took approx 10-15 mins, I followed the directions exactly.