We have seen the same issue too. Posting here to follow discussion.
AADSTS50011 redirect URI error when trying to sign into outlook.office.com/tasks
We have some clients that are trying to utilize shared tasks via outlook tasks/Microsoft To Do.
All of the users have Microsoft 365 E3 licenses, and Power Automate licenses.
When one shares a task with the others, it sends them a link that opens outlook.office.com/tasks and asks them to sign in with their microsoft account. After they sign in, they are getting the following error:
AADSTS50011: The redirect URI 'https://outlook.office365.com/tasks/auth/callback' specified in the request does not match the redirect URIs configured for the application '3ff8e6ba-7dc3-4e9e-ba40-ee12b60d6d48'. Make sure the redirect URI sent in the request matches one added to your application in the Azure portal. Navigate to https://aka.ms/redirectUriMismatchError to learn more about how to fix this.
I understand that the way you should typically resolve this is by going into the Microsoft Entra Admin Center and finding the application then adding that URI, but the problem is there is no app corresponding to the GUID given in this error in the Azure / Entra admin center at all. When I search "3ff8e6ba-7dc3-4e9e-ba40-ee12b60d6d48" it finds nothing, and if I manually look at each individual app, none of them correspond to this ID.
How can I resolve this problem?