Official Stock ROM, K50-T5_USR_ST1614_5.294.1_1604011150, Android 6.0, VibeUI V3.1
Download -
HERE
Unpack, use SPFT from archive, load, press download, attach connected phone to cable.
SPFT should look like this, i downloaded, unpacked and loaded, all works normal.
p.s. If you still receive same error, try another Download Agent from SPFT folder (ie: DA_SWSEC.bin or DA_SWSEC_CRYPTO20.bin)
b.r.
Alex