I have the same issue ... s6 sm-g920F. android version 6.01. I woke up to a frozen update screen for something involving knox (i think) two days ago, with phone frozen on a 'downloading... do not turn off target' message, with unit itself being quite hot. I exited frozen state by pushing vol down, home button and power simultaneously. now phone can pnly charge via usb to my pc (weirdly not to my mac even, only my pc tablet).
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