So I called Avis (in Oslo) to postpone the rental by one day. They obviously had problems changing the dates and ran into some problems. After speaking to both the office in Oslo and the national reservation agency (the local office didn´t pick up) we got the same message that the local Avis office (at the airport) was shut down until June!!!
Can anybody shed any light on this? Basically I don´t have a reservation now, and I really wonder how this could happen without me being informed about this.