Seems not related to this forum but there can be multiple reason for this , you can try out below:
Clear cached credentials
Go to Windows Credential Manager -> Windows Credentials
Remove any entries related to: Office 365 / MicrosoftOffice16 , Dynamics CRM
Restart Outlook.
Sign out/in from Office
In any Office app -> File -> Account -> Sign Out
Close all Office apps , Sign back in with their primary work account.
Check for multiple work/school accounts
If the user is signed into Windows with a personal Microsoft account and a work account, token conflicts can occur.
Remove the unused account via:
Settings -> Accounts -> Access work or school
If your organization recently changed MFA or Conditional Access settings, the user may be looping because their device is not fully registered or compliant.
Zscaler (or any secure web gateway / proxy) can impact this kind of repeated sign‑in prompt , check if you are connected with secure web gateway.
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