I had this error occurring in ILM when provisioning users in Live@edu.
The onPremise attribute flow to OnPremiseObjectType is called IAF-OnPremiseObjectType. It takes many values from the source, but I had set it to just use ‘mail’.
Turns out the objects reporting the error didn’t have a ‘mail’ attribute, so the attribute flow rule never got called. I changed ‘mail’ to something all of the objects have; ‘samAccountName’ and voila! Happy objects in the metaverse.
Hope that helps someone else.