Compiler error

Dear Sir, I am using Domjudge to set up ACM Contest in my school, everything is OK. I can submit Java, C, C++ very well. I also install cgroup and using chroot. But sometime, I received error like this when I submit any code, just somtimes this happened, and when I rejudge them, I received CORRECT response. Error text: Compiling failed with exitcode 255, compiler output: /domjudge/judgehost/bin/runguard: illegal user specified: 7806 Try `/domjudge/judgehost/bin/runguard --help' for more information. Can you show me how to solve this problem? And I also run domjudge without root accout, I run it with an account with root privillege! Thanks. I'm looking forward for your help! Best regard, Linh