How-to Fix An Unknown Error (1603) while updating iPhone firmware. The iPhone “iPhone” could not be restored. An unknown error occurred (1603). Fix 1603 Unknown Error by following these steps: Download iReb latest version iReb can fix 1600, 1601, 1602, 1603, 1604 error on iTunes. Run iReb iReb will kill (close) iTunes software and guide … Continue reading How-to Fix An unknown error (1603)