I'm also having the same issue. I did read the thread above but it did help.
I installed nagios with the following instructions:
nagios.sourceforge.net/docs/3_0/quickstart-fedora.html
To start, i'm a bit of a noob and havent used LDAP before.
Nagios installed and ran fine for a while, then I went to restart it, I got an error saying the nagios user didn't exist. I recreated it with the adduser command and it started again.
It happened again today, thought I would look for a perm solution and add the user via the web config tool. The problem is, I need a group called nagios and a user nagios, but the config script keeps erroring. I create a user, then a group, but I get an error on the group creating saying a user with the name already exists. vice versa when I create a group first then a user.
Any ideas?