Just so that you know this. I ran into this error when I opened the App Store:
Then I checked the URL being used to retrieve the XML data:
Looks like someone responsible for this at Apple forgot to put this on his calendar, to renew this certificate in time. Not so good.
A new certificate was issued by Symantec yesterday, and thus this issues is resolved.
I’ll be waiting for the next Apple certificate to expire – I found one that is about to expire as well. Anyway. Now we know that developing a Calendar.app is anything but helpful… if nobody is using it 😉