Hi @Abhishek Negi ,
I am also able to replicate this issue with new password change scenario of signed-in user in Microsoft Graph Explorer app. As you can see below, I did tested the same using same user's account in Postman API tool with the delegated scope and List chats API is working fine without any issue.
Moreover List chats API is retuning the response in graph explorer for some times when you re-execute the API again and receving "Unauthorized" error response for some times. Graph exporer runs based on the deleged scope with signed-in user's account and it seems to be an intermittent issue with graph explorer azure AD app because of new password change. My understanding it that, it may take sometime for graph exploer app for auto resolution. Please verify it again in day or two and see if it is resolved.
In the meantime, I would also advise you to use Postman with the Microsoft Graph API as an alternative API testing tool.
Example output in Graph Explorer app:
Example output in Postman API tool :
Hope this helps.
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