Surely the computer you’ve initiated the backup from?!
The message pops up when I put the iPhone on its charger.
I do not know at the time if the Mac has initiated the sync or not as it is in another room.
I would have to go back to the Mac, wait for the "cannot initiate sync because the device is protected with a passcode" message to appear and then manually click sync again.
That is a a hassle and it is supposed to be easy.