found this post from 2012:
The negative value of ESR is some error, it hasn't been investigated fully yet, but there's been some talk on the topic, some time ago here on the forum, and someone has found that one of the Tantalum capacitors in the circuit affect this behaviour, more precisely the esr of that exact cap.
Though if you zero the instrument, it should go to zero, and stay there at least till you switch it off and on again...
So the negative value is because the esr of the measurement circuit of the instrument is drifting... and because, the reading is computed using a previously saved calibration value, this drifting can make the final measured value negative on the display...
nd this post from 2012.