I could find a solution myself. 

You have to go to "Bank Stripe", make sure the serial number of the terminal is correct and also that the terminal is set up for a location in Stripe. Then go to Stripe Connect -> and there you have to make the Stripe connection. 

This is all what I already did. But I found out when you have the error that I have you have to manually type in the private and public key and create a webhook. But there was no option for me to do so. So I went into developer mode and came back to the screen after you go on Stripe connect and then BAM you have the fields to type in the keys and do the webhook and when you did, it's working.

It's very stipid that there is no easier or more obvious solution but at least I found a small workaround