Ah, so your earlier "my bad" message meant to say that nothing is actually wrong with the +/-10 rails. Got it.
"Instead of checking voltages between chassis ground and directly at both emitters, I probed between ground and + C215/ C218. According to the schematic, this should be the same. But apparently not"
You MUST get the same values at the Emitters and at +C215 and -C218 ! If you don't, you've got a measurement problem, OR those caps are not actually connected like they should be.
Note that those rails *may* appear to work fine, even if C215 and C218 are missing...