exec error ("A specified logon session does not exist. It may already have been terminated.")

I'm trying to use exec (or system) to create NT user accounts on our domain. My plan was to add a user on the command line with the "net user /domain" command. This has to be executed on the PDC (NT 4 service pack 6) rather than on the web server (which is 2000, but I'm not sure that matters). When I first started, it seemed to work just fine. Now, without changing the script, I'm getting the following output: A specified logon session does not exist. It may already have been terminated. before any PHP code gets to the browser. This now seems to happen regardless of the command I'm trying to exec, and only goes away when I comment the exec line, which of course doesn't do me a lot of good. Does anyone have any insight as to why this happens? thanks.