Hi Ludwig,

By looking at the error message, I can follow up that that you have an issue with creating Microsoft Account and receive error message "We’re sorry, but something went wrong. This user wasn't added to this PC".

You can try the below methods to resolve this issue:

Method 1:

You have to make sure the Email ID that you are using to create Microsoft Account is not associated with more than 5 computers.
Windows 8 has a limit on the number of devices that can be associated with the Windows Live ID for login into the account.

Method 2: Run the Microsoft Account Troubleshooter

Windows 8 have developed a troubleshooter to address several specific Windows 8 synchronization issues.
This won't help for issues where you have forgotten your password, or your account has been compromised.

Method 3: Setting correct date and time

The problem could be because of the date and time beingwrong on your device. You should try to update the date/time and the time zone, and your existing Microsoft account may be logged in normally.

To set the date and clock you can click on the Time and date in your lower right screen on the desktop. and then click "change date and time settings."

I hope this helps in resolving your issue.

Regards,
Lokesh