[Crayder]

pawn Код:

if(!strcmp(hostname,Server[validhostname],false))
    {
        format(szCmd, sizeof(szCmd), "hostname %s", Server[validhostname]);
        SendRconCommand(szCmd);
    }

In this 'if' line you are checking to see if the current hostname matches Server[validhostname], instead of the opposite which is what you need. You'll want to remove the '!' to make sure the strings do NOT match.