Our solution seems to work great:
3xVictron Multiplus II 5000VA + MPPT 150/100 + Fronius Symo 20.0-3-m (10kWp installed, MG50 configuration).
I use ESS and AC ignore based on generator start/stop function (Cerbo GX relay is connected to L1 aux). When the load goes over 4000W then grid is connected as a backup and overload prevention.
When a Fronius is producing e.g. 2000W on each phase, this production is added to AC load sum, because it is being consumed, which I think right.
BUT it also triggers a condition "Inverter AC out highest phase". However the real AC output is 2000W less, because of Fronius AC production.
Currently I solved this by telling CerboGX that the Fronius is located on AC-in, so it does not sum up its production to AC load of the system.
I think that proper solution would be that the condition "Inverter AC out highest phase" takes into account "real AC out power", but not "real AC out power" plus Fronius production. Because the production of Fronius does not even go to the Victron MPII and is being consumed inside the microgrid network and there is no overload of Victron MPII.
