Hello everybody
This is for all the people who can not arrange a pick up time on the return portal after they got a confirmation mail hat their return request had been approved.
The problem:
I made a return request on the return portal and got a confirmation mail, that my request had been approved. It contained the shipping labels and asked me to chose a pick up time for the DHL currier.
When I clicked on “book a pick up time”, I got directed to the return portal on which I got the following message:
An error occurred. Booking not allowed for this return.
I also got the same message when I tried to book a return directly on the return portal.
The solution:
I called fairphone today and apparently there are some problems with the return portal and customers from certain countries (Switzerland in my case), can not chose a pick up time. Fairphone is working on it. But since I had the confirmation mail and the shipping labels, I was told, that I could arrange the pick up time directly with DHL. All I needed was the FRT number, so that DHL knows whom to send the bill.
You can find this number on the second shipping label. Right under the big country code, it says Payment code: FRT A/C: followed by a 9 digit long number that starts with a 9.
I called DHL today and it really was no problem. I was asked for an invoice number and gave them that FRT number. And now they are picking up my parcel tomorrow. By the way, they also wanted to know the weight and the volume of the parcel, for which I was not prepared. So think about that before you call them 
Hopefully this will help other people who are facing the same problems.
Edit: The DHL currier also wanted a commercial innvoice, which you can easily fill out here