Not that long, more or less 2 meters. It is an original Victron USB-cable. I assume that it is a twisted pair cable.
It is installed next to AC-power cables going to the mains supply over a distance of only 80cm’s. It is not close to DC-power cables. The Victron wiring guide has no topic on RS485
If the RS485 suffers from vincinity to AC-power, why only during charging? During discharging the current is in the same order of magnitude without any disconnection ever. The direction of the energy flow (positive or negative) can not be a factor, can it? Or when idle at a sunny day my 10kWp AC-out connected PV-panels are supplying to the grid even more power at higher currents via the same AC-cables.
The problem resembles Em24 disappears in device list and system goes in passthru mode In this case the grid meter is connected via modbus/TCP instead of USB.
It also resembles Quattro going to Passthrough when it should be Charging In this case the internal grid meter of the Quattro is used.
The resemblence could be a red herring of course.