Here are some more findings about the smart home rule about when the AC-in contactors should be closed:
I found out that forever reason, in case if the inverters have less than 100-150W load each, at the moment when the AC-in contactors get activated to connect them to the grid, sometimes (about 2 out of 10 times) some but not all go in Fault 60 status (grid feedback protection). Because of that, I've changed the smart home rule a bit to not only check the SOC (or voltage) of the batteries to activate the AC-in contactors, I also check the current load of the AIO's. In case the load is less than 150W for each AIO, I activate a large 240V load, wait about 10 seconds to balance the load between all the AIO's, switch the AC-in contactors to ON, wait again some seconds and deactivate the large load. This solved the sporadic problem with the F60 error! In my case the "large 240V load" is a pump which I can remote control anyway.
It looks like the design of these Voltronic, Axpert Max (MKS2-6500) based AIO's are in general able to feed energy back to the grid - but in most clones this feature is removed in the firmware (like in the EG4-6500EX - I know at least one of these clones which has the grid-interactive feature available for configuration, it's called Phocos AnyGrid model PSW-H-6.5KW-120/48V). I don't really know, why this F60 only happen if the inverters have low load at the moment the grid (or a generator) will be connected. If your connect them to the grid while having idle load, the problem occur every 2nd try, so it depends really on the load! After changing this, I've never seen this problem again (about 20 test cycles so far).