Permissions on files have caused about 50 problems in this forum alone. So check all your folders, all your files, for permissions problems.
In particular, check your objects.cache file, since it looks like you are using the beta version of nagios v2.0. And if you are, I have to ask why? Unless you are a nagios expert, why would you want to start your nagios experience by troubleshooting all the bugs in the beta? My suggestion is start with something NOT buggy, v1.2 and if you get bored and want to troubleshoot a beta, then go for it later, but not now.