I am not sure about the Aerzen blowers but most brands do not have a proper seal between the gear box and the blower chamber but rely on close tolerances and labyrinth arrangement.

My only suggstions are:

1) Check that you are using the correct grade of oil.

2) Check that the gearbox is vented to atmosphere so that any build up of pressure is vented out rather than in.

3) The oil levels are correct as in not overfilled.

4) It will probably mean a strip down but a check of tolerances on the shaft and seals might be worth it.

Regards

TerryF