Thermomix TM31 shuts off during start

hi all,

i'm trying to repair a thermomix TM31 which shuts off during boot every ~0.5 - 1 seconds. it's stuck in a boot loop. the screen flickers sometimes. so i'd say, it doesn't get enough power and then restarts. very few times, it actually boots normally but then shows error 52 which tells us that there's a connection issue to the motor.

what i did so far on the PSU board: i replaced a triac that was totally fine ( :D ) and i replaced all 4 opto couplers because one had a bit more resistance than the others.

the motor connectors are not shorted.

i checked for: shorted capacitors, shorted mosfets, shorted bridge rectifier.

from the error i described (boot loop, flickering screen) maybe you have an idea at what component i could look next. i attached a picture of the PSU board (front/back).

i'm a (motivated) newbie, so please be patient with me :)

thanks, narf