Strong (power) oscillating 3 phase MP2 installation (up to 2kW swings) @0,5 Hz

I notice your voltage also seems to be swinging with the same frequency, by 3 or 4 volts which is a lot for 500W (or 2a). Is that a cause, a symptom, or just coincidence?
Also, in other oscillation discussions (e.g. this) it was suggested to look at the dbus round trip times in VRM; in my installation, that is in the single digit ms range.