Enabling SQM causes bufferfloat to go from B to C on a cable connection

I fear your are not the only one having problems with traffic shaping on 860-L. Unfortunately it is not clear where exactly the problem lies, but it can be triggered by instantiating traffic shaping. It is not only sqm-scripts, but also qos-scripts that can trigger the router's susceptibility, not that helps you but see:
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Best Regards