Small update.
The AmigaExplorer (AExplorer)-Software is a mess in combination with an USB2Eth and a RapidRoad.
I used to use an asix-chipset based device. AExplorer just won't work in this combination.
I have started to try an old rtl8950-based adapter. After setting the packetsize to 65535 (max) it works ok but still with issues.
I will investigate some further.