The parts arrived from Mouser.
I replaced U2101 (DAC), U2521 (4051) and U2420 (TL084) as well as all the 0.47uF capacitors connected to the U2521 just in case.
I though I read somewhere the the capacitors in this circuit may be the cause of having low +1.36V/-1.25V values, as I was experiencing (is it true??).
Anyway, I have now the error changed to TEST 4 FAIL 2 but voltages' prior to DAC adjustment are +1.4V and - 1.28V !!!
It seems I will be possible to execute the DAC calibration and move on to the other calibration steps.
Unfortunately I have to travel tomorrow and will only br able to work on the scope on next Monday.
Thanks to all of you that have helped me to get to this point.