This frustrates me that hosting your own next cloud still has advertising to sign up for next cloud when sharing files with users. It shouldn’t be set as default for people who host. To be honest Nitrokey should have disabled this before shipping. Below is the solution to remove it.
I already contacted the support team of nitrokey about this, as it really annoys me, too. I got told that they most likely will remove it in the future because it’s not a big change in the nextcloud system itself which the acutally try to touch as least as possible.