User not automatically logged in when movong to checkout url.

[Dr-Pixel]

Hey,
I have a problem related to Customer Account API. I have the Single-Sign-On implemented in my Hydrogen storefront and it works fine and like expected. When the user moves to the checkout he is not logged in. But if he clicks "Log in" in the checkout he will be logged in again (without receiving new E-Mail). Why is he not automatically logged in like it works in the hydrogen.shop.

I'm looking forward for help!
Thanks a lot in advance.

[Dr-Pixel]

Found the problem:
I did not enter the the checkout Domain in my callback-urls and javascript regions in my hydrogen storefront emviroment.