[bojan]

Faulty FET is replaced with new one and supply voltage to the front board (which controls shutter and flash) is now present.
However, error 99 didn't go away...
Upon pressing the shutter button, I think I felt something was trying to move (mirror motor?), and there is electrical activity on motor terminals, voltage drops from VBAT to GND, and after that - error 99.
It seems the logical next step is checking the motor and overall mechanical status of the mirror box. Manual manipulation of mirror looks normal...

Another symptom surfaced now (I haven't check that before): Autofocus doesn't work when lens is in place, and there is no activity either... but now there is no error 99.
It seems the problem (which also caused FET to malfunction) is further down the track.