Well, I didn't mention I used italian MiFlashPro (androidfilehost.com/?fid=4349826312261688231), sorry. In that version, I didn't get the auth error. Please try that. However I got the error code BROM ERROR: STATUS_PROTOCOL_ERR (0xC0010005), which means that SP Flash Tool can't communicate with the device. Solution: check if the phone is properly switched off. I had to drain the battery to make the phone turned off.
Edit: Win10, Hungary