SSO for shopping doesn't work

I'm trying to place an order, already on many browsers (safari/firefox/chrome). No matter the browser, I never can go through the 'Sign-in via CCP' step in the cart checkout process.

I get the following error by inspecting the request to https://www.netcup.com/api/shop/auth/singleSign address:

{
"success": false,
"errors": [
{
"code": "authenticationError",
"message": "Authentication token is not a bearer token."
}
],
"messages": [],
"meta": null,
"result": null
}

When redirected to the CCP login, I get a Single Sign Key is not valid. error after logging in:


How can I proceed with my order?