Hey Gerhard! Nice to meet you!
Based on your description and the error message you provided, kindly perform according to Microsoft Community Moderator | Bill: https://learn.microsoft.com/answers/questions/4724989/error-400-when-signing-and-do-not-have-any-other-a?forum=outlook_com-all&referrer=answers
"Please ensure you are connected to the Internet and the connectivity is fast, strong and stable to avoid any authentication errors and successfully log in and access your Microsoft account.
Kindly try to log into and access your Microsoft account (switching between Outlook.com Webmail, Outlook client app, or Outlook mobile app) from a different device using a different network such as Work or Home network and even use a different Internet connection such as an Ethernet cable or network, Mobile (Data) i.e., Cellular network or Wi-Fi network or a different Wi-Fi network.
Try to restart your Ethernet or Wi-Fi connection modem and router, as sometimes, a simple restart can resolve temporary glitches.
Also, please check with your Internet Service Provider (ISP) to see if there is any issue on their end. "
If the issue persists, kindly download the Microsoft Authenticator app and add your account. This step has reportedly resolved the issue for other users.
Download Microsoft Authenticator app: https://support.microsoft.com/account-billing/download-microsoft-authenticator-351498fc-850a-45da-b7b6-27e523b8702a
How to add your accounts to Microsoft Authenticator: https://support.microsoft.com/account-billing/how-to-add-your-accounts-to-microsoft-authenticator-92544b53-7706-4581-a142-30344a2a2a57
Thank you for your kindness and understanding! Please feel free to let me know if you need anything else.
Best regards,
Darhyl