Currently log.user() only works within job_submit(), and does not currently work in job_modify(). Commit 3f4cde9c changed job_modify() to throw an error if log.user() was used. It be nicer long-term to permit this, and return the message back to the user instead.
This was already raised in bug 4391. *** This bug has been marked as a duplicate of bug 4391 ***