Hi Jan,
Thanks for your update.
Based on the error message, there’s still a method you can try by your own: Sign up for an account on the Office 365 website.
If the process fails, perhaps the only way is to contact your administrator directly to ask him to assist you in changing your password. Sometimes, when using Office 365 for Business, if you encounter some server-side problems, we need your Global Admin to
go to the Office 365 admin center to open a support ticket. Our back-end support engineer would solve the problems from their perspective. If you don't know who your admin is, you can ask your organization's IT department for help.
Appreciate your understanding.
Best regards,
Madoc