Back feeding when connecting one side of circuit to differential op-amp
Really, that circuit doesn't make sense.

Leave SW1 open and remove all the components that don't carry any current anymore.

You'll notice that OA4 needs to make the voltage between its two inputs 0V (golden rule of opamp with inverting feedback), and since the positive input is pulled to ground (no current in our out of inputs, hence no current through R27, hence no voltage over R27), the output of OA4 needs to become 0V to. It tries to, but I'd guess it has a minimum output voltage of GND+0.66V.

Also, notice that you're feeding -30 V into that OA4. It's voltage supply range is 15 V – that's a recipe for disaster.

So, I don't know how to save that schematic. Design it from scratch, anew?