Dear SvenW,
I’m seeing very similar behaviour on my 3-phase system after updating to GX v3.66 and Multi firmware 510.
In my case L2 and L3 remain in import / pass-through even though voltages, CTs, Dynamic Cut-Off, Assistants and the Pylontech US3000C BMS are all correct.
The system shows no VE.Bus error, and the behaviour is stable until I remove all Assistants — then everything works normally again.
Once the Assistants are added back, L2/L3 go into import again.
My second installation, with the same hardware and the same Assistant setup, but still running firmware 502, works perfectly — so the issue may be related to the newer firmware or ESS logic, similar to what you described.
I will reconfigure everything from scratch and also check VE.Bus configuration with MK3 next week.
Would be great to hear if you observe the same behaviour on GX 3.66.
Here is link to my post