I appear in the system as an unknown user and my colleagues can't see me, even if they are in the chat.

Hi @Дмитрий Басков, 

I hope you are doing well. 

Thank you for reaching out and sharing a detailed description regarding the issue. Based on Microsoft official article, the issue can happen when your account was disabled and re-enabled since Microsoft Teams and other Office 365 services may lose track of the user’s profile information. 

To resolve the issue, please follow these steps ( The following guidance is for Windows devices. If you are using a different device, please let me know.): 

1. Clear Teams cache 

• Close Teams completely.
• Open the Run dialog box by pressing the Windows logo key + R. 
• In the Run dialog box, enter the following path, and then select OK:   %userprofile%\appdata\local\Packages\MSTeams_8wekyb3d8bbwe\LocalCache\Microsoft\MSTeams  

• Delete all files and folders in the directory. 
• After that, restart Teams  

2. Sign out then sign back in 

• Select your initials or profile picture at the top of the app. 
• Select Sign out. 

• Sign into your account and see if the display name is back to normal. 

For more information, please refer to this article: User appears as Unknown User in Teams - Microsoft Teams | Microsoft Learn 

I hope this information is helpful. Please follow the steps above and let me know if it works for you. If not, we can continue working together to resolve the issue.  

Thank you for your patience and understanding. If you have any questions or need additional assistance, please don’t hesitate to reach out so I can continue to support you. If you found the response useful, please consider marking it as accepted, as this may help other community members who are looking for similar guidance.  

I look forward to continuing the conversation.

If you have extra questions about this answer, please click "Comment". 

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In Microsoft Teams, a user can appear as Unknown User in these cases:

1. The account was deleted
   • If the user account has been deleted from the organization, all past messages and mentions from that account will permanently show as Unknown User.
   • This is by design and cannot be reversed, even if a new account with the same name is created.
2. The account was disabled and then re-enabled
   • When an account is disabled and later re-enabled, the display name may temporarily show as Unknown User.
   • This usually corrects itself automatically within about two weeks.
   • To speed this up, the affected user should:
     1. Sign out of Teams on all devices.
     2. Sign back in.
     3. Allow up to 72 hours for the display name to update.

If the account is currently active and not deleted, ensure sign-out/sign-in has been done and wait up to 72 hours. If the account was deleted at any point, the Unknown User label for historical content is permanent.

References:

• User appears as Unknown User in Teams