This question has been answered by gissirob. See the first response.
There is a known bug that generates this message any time a user is deleted. It is possible that this occurred for some other reason, but I would check there first. Supposedly it is harmless, but it would be nice if it was fixed (it's been around for a number of years).
Quote from: gissirob at Feb 17, 2015, 10:09 AMThere is a known bug that generates this message any time a user is deleted. It is possible that this occurred for some other reason, but I would check there first. Supposedly it is harmless, but it would be nice if it was fixed (it's been around for a number of years).
And it's still there.
For me it was I needed to set primary_group for modUser!