Next update:
After practically everything else seemed to check out I've moved one of the
original opamps to the clone board.
And lo and behold - the oscillation went away.
Originally I had been using an MC4558L; I have also swapped this particular
one against another MC4558L with the same result (self oscillation).
Did the swap to the original opamp on the second board as well - same
result, self oscillation gone.
This was IC15; I'll check later or tomorrow what happens if I also swap IC12.
After practically everything else seemed to check out I've moved one of the
original opamps to the clone board.
And lo and behold - the oscillation went away.
Originally I had been using an MC4558L; I have also swapped this particular
one against another MC4558L with the same result (self oscillation).
Did the swap to the original opamp on the second board as well - same
result, self oscillation gone.
This was IC15; I'll check later or tomorrow what happens if I also swap IC12.