Hello,
We tried to update the theme and did not face any problem with that https://gyazo.com/334e2825d37a1d883ef066faf85c7916
As a rule, such issue appears if you activated the theme on another domain and then moved to the new one. Then you need: 1) reactivate the theme using your purchase code 2) Click the Check again button to clear the old token http://prntscr.com/vycwqx and then press the update button.
Regards