I have same problem!! purchased my ipad from Hong Kong official website in 2012. Updated ios 2 days ago and my ipad is now locked by an unknown apple id kxxxx@1xxxx.com!!!
spent whole day with Apple Support today, but they cannot find my invoice back in 2012. And they said they cannot help me to unlock without presenting the original invoice!!!
But my apple id shows this device is linked to my id!!! But support says "that doesn't count only icloud's apple id counts" as ownership proof!!! I am helpless! I mean this is apple's security issue and my icloud is hacked but they refused to help!!!
I really don't know what to do !!!
So ridiculous they want me to check my 2012 credit card statements to proof i purchased this ipad! Who would keep such records???
"Maybe you can call your bank to check?" they said. I don't even remember which credit card i used!!!! The weirdest part is they can check serial number of this ipad is linked to my apple id, yet they said that "doesn't count" ! I really don't understand what counts!!!
I have been using iphone and ipad since their first production. This is truly disappointing. I wish someone here can help, or RIP ipad.