I am having these issues with 2 systems right now. The Victron is shifting frequency, and it is locking and keeping the PV inverters off. One system it happens at 60%, and in the other, it seems to happen once the batteries are full, and it won't shift frequency back to 50 Hertz to turn the PV inverter back on when a large load is applied.
Victron Inveters are 15kva 48v Quattro. And a 5KVA Multipluss 48v. Both PV inverters are Fimer 5kw ac coupled. I have used this combination over 100 times and have not had these issues. To be clear these are 2 separate systems.