Hi @Roman Seidel ,
In my experience, the email address you use for your Microsoft Account can be from outlook.com, Hotmail.com, Gmail, Yahoo or other providers. We recommend you try to change another provider to see if you can create Microsoft Account normally.
You cannot add custom domain without Microsoft 365. If you want to add your own domain to Microsoft 365, you can learn it from:
https://practical365.com/microsoft-365/configure-a-custom-domain-in-office-365/
https://learn.microsoft.com/en-us/microsoft-365/admin/setup/add-domain?view=o365-worldwide
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