usermod: Can't modify user

[Roel Sergeant <rsergeant%gmail.com@localhost>]

Hi,

I've upgraded from 4.0.1 to current recently, but now it seems I can't
modify my users anymore using usermod. I get the following error:

usermod: Can't modify user '...': must be a local user

I'm just trying to put the user in the group wheel. There are two
things I don't understand:

1) Why is my user no longer a local user? Did I make a wrong
modification during etcupdate?

2) How can I modify my user when it is not a local user?

Thanks in advance,
Kind regards,
Roel.