No, from the can of the motor back to the negative terminal of the battery, not the negative terminal of the motor, that would create a short circuit condition.
In your original post it said you'd disconnected the white wire from the ESC, which is the control pulse not the red wire.
So no servo fluttering with the ESC disconnected, even when operating the piston tank. Also no fluttering either when the Unipro is disconnected and the ESC is hooked up. What happens if you hook everything up with the ESC's BEC powering things? e.g. no external pack.
It sounds like some sort of clash between the ESC and the Unipro board. These kind of faults can be tricky to nail down to be honest. I had a problem with two speed controllers of different manufacturers that were fitted in a submarine. One controlled the pump for the ballast system, the other the drive motor. Intermittently the signal failsafe would kick-in for the pump ESC, leaving me with a boat that either wouldn't submerge or surface. I tried every trick in the book, in the end I replaced one of the ESC's so they were both the same- end of problem.