At the moment, the lookup of a user picker field is by ID and not text, so when a rule tries to populate the field A4J shows a message saying "Inactive user" even if the user is active:

And as for the logs, the message we receive is:

Response body: {"errorMessages":["Specified user does not exist or you do not have required permissions"],"errors":{}}

 
That said, changing the message error in the Audit Logs to be more accurate such as "You can only populate a user picker field by ID".