Hi @Christine Chevrier ,Currently in Microsoft Q&A we only support English, could you please use English in the following description? Then we can help to solve your issues. And we are researching based on machine translation, I understand that you are getting an error when trying to log into your Outlook application, if there is any misunderstanding, please let me know.
To narrow down the issue, please provide detailed information:
- Does this problem also occur in other Office applications?
- What type of account are you using?
Here are some steps you can try to resolve it:
- Repair Office: Go to Settings > Apps > Apps & features. Find Microsoft 365, click on it, then select Modify. Choose Quick Repair or Online Repair and follow the instructions.
- Create a new profile and re-add your account to see if it can be fived: Go to Control Panel > Mail> Show profiles, delete the old profile and create a new profile, then re-add your account.
- You can refer to the comments in this link to see if they are effective for you: i've got this tag error 4vt9f in outlook application - Microsoft Q&A
Also, is the email you mentioned about adding a professional domain an email from a custom domain? If so, you may want to Choose a Microsoft 365 Plan to achieve your need.
More detailed guidance, you can refer to the How to buy a domain using Microsoft 365 Business Premium
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