did you delete the plugins/admin folder?
It seems that this is not a fresh installation, there are some problems if the hostname can not be resolved during setup so I’'m not sure whether this applies also for you:
“Make sure that the unix command “hostname” returns a valid name, for example “mycastle” and that your /etc/hosts file does contain this name, for example in the first line together with “127.0.0.1 localhost mycastle”. Or that you can resolve it using DNS. Actually the setup page after choosing your locale settings will return a HTTP 500 error if there is a problem.”