I have a workflow that runs as you'd expect -- until I tell it to update a
person field on the item in question... with a particular FBA user. This
site knows how to access the forms auth provider, and the users even show up
in the picker in SPD. However, when I select the appropriate FBA user and
run the workflow, I get the dreaded "Error updating a list item" message.

Does anyone know why I can do this with an AD user but not with an FBA
user? I've checked to make sure it isn't a security restriction problem
(the FBA users are in the proper group in order to pass validation), and I
can't think of anything else that could be going wrong here.