Not sure if I was having some kind of other issue but this would not work for me doing it exactly as you have documented and I am wondering if this just slipped out or if its actually required and missing from the documentation. 
Easy to miss actually so I am betting that is the case but for anyone else's benefit who might try this and end up with the same thing I was getting I'll add a bit more detail and explain my setup a bit.
What was happening to me was in trying to hit 
https://admin.example.com or 
https://admin.example.com:8443 or 
https://admin.www.example.com:8443 in chrome I would get an error message saying:
 This webpage has a redirect loop
The webpage at 
https://admin.www.example.com:8443/ has resulted in too many redirects. Clearing your cookies for this site or allowing third-party cookies may fix the problem. If not, it is possibly a server configuration issue and not a problem with your computer.
don't recall the firefox error message. 
I would also frequently get a url that looked like 
https://admin.www.example.com:8443/admin...x.phpadmin or something like that and in my apache error logs I would get this:
192.168.1.145 - - [15/May/2011:22:24:53 -0700] "GET /admin/index.phpadmin/index.phpadmin/index.phpadmin/index.phpadmin/index.phpadmin/index.phpadmin/index.phpadmin/index.phpadmin/index.phpadmin/index.phpadmin/index.phpadmin/index.phptools/webmail HTTP/1.1" 301 322 "-" "Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.2.17) Gecko/20110422 Ubuntu/10.04 (lucid) Firefox/3.6.17"
What resolved it was modifying the basedir restrictions in the 00_master.conf so they read as follows:
<Directory /var/www/ispcp/gui>
            php_admin_value open_basedir "/var/www/ispcp/gui/:/etc/ispcp/:/var/run/ispcp.lock:/proc/:/bin/df:/bin/mount:/var/log/rkhunter.log:/var/log/chkrootkit
.log:/usr/share/php/"
            php_admin_value session.save_path "/var/www/ispcp/gui/phptmp/"
            php_admin_value upload_tmp_dir "/var/www/ispcp/gui/phptmp/"
        </Directory>
What is different than what you have in the documentation is the inclusion of /var/www/ispcp/gui: in the first stanza. It was in the original and yes you are calling it out in the <directory  line BUT I believe you are also basically excluding it by not having it in there in the actually open_basedir directive.
Adding that for me resolved it and got all my control panel pages accessible again.
Perhaps cause I am using mod_gnutls and mod_ssl but at least for me it wasn't working.
I was tested this out on a dev system before I make the changes to my other systems.
Here is the system config if it might help someone else.
lsb_release -cd
Description:    Ubuntu 10.04.2 LTS
Codename:       lucid
PHP 5.3.2-1ubuntu4.9 with Suhosin-Patch (cli)
apache2                              2.2.14-5ubuntu8.4 
apache2-mpm-worker                   2.2.14-5ubuntu8.4                             
apache2-suexec                       2.2.14-5ubuntu8.4                             
apache2-utils                        2.2.14-5ubuntu8.4                           
apache2.2-bin                        2.2.14-5ubuntu8.4   
apache2.2-common                     2.2.14-5ubuntu8.4   
libapache2-mod-fastcgi               2.4.6-1                                        
libapache2-mod-fcgid                 1:2.3.4-2ubuntu0.2                       
libapache2-mod-gnutls                0.5.5-1  
openssl                              0.9.8k-7ubuntu8.6
ispCP:
BuildDate = 20101124
Version = 1.0.7 OMEGA
CodeName = Priamos
DistName = Ubuntu
Reading my error logs and the rewrite logs was not as helpful as just comparing line by line to a working non-modified 00_master.conf file. I even reworked the structure of the file a bit so it was more readable to me, that was when I notice the one line missing.
Hopefully this helps someone else out.