I know that to update a person or group in a list using the UpdateListItems web method, I have to have the user's ID, a semi-colon, a pound sign, and then the alias:
And if I don't know the ID, I can use -1 and ShP will look it up for me:
However - it doesn't seem like this would cause the issue with your workflow - since it works manually but fails when it runs automatically. Any info on the failure?
I'm not surprised by the message on the property promotion - I don't think IP has a data type equivalent to person or group (even though the people picker likes to pretend)