We're trying to call the standard SharePoint 2010 web service UserGroup.GetRoleCollectionFromUser. No matter what user we use to authenticate, the web service always returns the same error:
"User cannot be found"
Of course the user does exist and the user name has the form DOMAIN\user. Other methods of the UserGroup web service work fine. In particular, UserGroup.GetGroupCollectionFromUser() with the same user name returns the expected results.
This web service works fine on 2007, although the user privileges required to call it are somewhat unclear.
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