Hi @CBOIN ,
Did you use the Single sign-on?
Did you use the shared activation mode for Office 365?
I suggest you go to Credential Manager- Windows Credentials to remove credentials related to Microsoft 365.
Then go to Registry Editor, locate to Computer\HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Common\Identity\Identities, delete Identities registry to reset it.
Serious problems might occur if you modify the registry incorrectly. Before you modify it, please remember to back up the registry for restoration in case problems occur.
Here is an article about how to back up and restore the registry in Windows: https://support.microsoft.com/en-us/topic/how-to-back-up-and-restore-the-registry-in-windows-855140ad-e318-2a13-2829-d428a2ab0692
Here is the similar problem thread, you may get any information from it:
https://answers.microsoft.com/en-us/msoffice/forum/all/office-365-on-rds-keeps-asking-users-to-activate/6f10a9b6-8f45-4062-a05b-217f87cb6b23
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