Apinto is correct, one thing to try is to go to Module Admin >Apache Config >Override A Virtual Host >select domain >un-tick 'OpenBaseDir' and save - wait ~5mins for daemon to run and update the vhost for that domain.
This disables the OpenBaseDir restriction for that domain. This poses a possible security flaw if the site gets hacked, but you will be able to run elgg as they suggest.
This disables the OpenBaseDir restriction for that domain. This poses a possible security flaw if the site gets hacked, but you will be able to run elgg as they suggest.