When using the Office365 Activity to authenticate the following error is shown: ErrorCode invalid_grant. Message = AADSTS70002 Error validating credentials or AADSTS50126 Invalid username or password
The error message is an authentication error returned by Azure, because is not recognizing either the user or the password. In the case of a managed user (user from an Azure AD tenant opposed to a federated user, which would be owned in another IdP through ADFS), the user has entered the wrong password.
The user should directly try to log-in directly on Azure with that Username + Password, if succeeds should check if is part of the right Azure Active Directory and if properly configured it on Studio.
If the log-in is also failing in Azure, the user should contact the Azure Administrator to reset the password or mitigate the issue for that particular account.