Long story short, I had some reasons to flash ST15i in past but had never seen this error before.
Log looks like that:
30/011/2018 02:11:11 - INFO - Selected Bundle for SonyEricson Xperia Mini(ST15i). FW release : 4.1.B.0.587. Customization : Unbranded
30/011/2018 02:11:11 - INFO - Preparing...