U201 is defective, because it doesn't regulate the constant current through the 28.75k hybrid resistor correctly, and its offset between + and - is way too high.
U400-A is defective, as it does not amplify correctly to 10V, (and its offset is way too high, also), which in turn doesn't feed enough current to the LM399, its output is probably oscillating.
So the -10REF voltage of -6.13V might differ that much from the +10REF value of 6.35V, due to the oscillation also, but U401-A might as well be defective.
This can be decided only after replacement of U400, when the +10REF is ok again.
You should order 3EA of the AD706.
This -14.4V supply voltage is probably ok, as the value of CR402-A and also CR401-A probably have been changed to 3.6V at some time.
Take notice, that the +15V supply is ALSO about 3.7V lower than these 18.73V, instead of the nominal 3.3V zener voltage.
Frank