Siebe said:“Nibbana is often described in a negative way. As somethings that are not present anymore, such as asava’s or craving or avijja.”
I think you meant to say, “Nibbana is often described as an absence of something. As somethings that are not present anymore, such as asava’s or craving or avijja.
That is correct. Nibbana is reached via getting rid of asvas and avijja, which is equivalent to getting rid of greed (lobha), hate (dosa), and avijja (not knowing about the real nature).
Siebe said:“Or must Nibbana also been seen as the summit of the presence of relative (conditioned) qualities like mindfulness, concentration, metta, mudita etc.”
Cultivating qualities like mindfulness, concentration, metta, mudita etc. are part of that process.
In other words, the way to Nibbana is the Noble Eightfold Path. One learns about the true nature of the world (Dhamma including Tilakkhana) and get to to Samma Ditthi. Then one starts thinking along those lines and cultivates Samma Sankappa, speaks accordingly and cultivates Samma Vaca, etc. See:
How to Cultivate the Noble Eightfold Path starting with Anicca, Dukkha, Anatta
Siebe said: “..still i do not understand why Nibbana is not of this world.”
So, from the above, it is clear that Nibbana is attained via getting rid of everything (attachment to everything) in this world, because that leads to suffering. Thus it is a tautology: Nibbana DOES MEAN completely dissociating from this world (even though that sounds harsh, that is the only way to permanently escape from suffering).
But it is not easy to comprehend this fully, even for one with lower magga phala. So, there is no point in trying to make sense of that statement. One just need to go step-by-step, to the next step where ONE CAN see the relief from suffering.
The first step to experience the relief of getting rid of excess greed, excess hate, AND cultivate metta, karuna, etc AND to learn Dhamma. This process is discussed step-by-step in the “Living Dhamma” section.