> I guess it was my time to fry a module. I lost an A-138V2 (linear) mixer by misconnecting it. If it’s likely to be the
> opamp, I’d be glad to give it a try, maybe replacing it with the OP470AY for kicks.

I'm quite sure replacing the quad OpAmp (LM324/TL084/TL074 compatible) will solve the problem.

Best wishes
Dieter Doepfer