Try to see if you have a "error_log" file that perhaps will give you a FATAL error message and will help you identifying where you have a permission error.

I will check with my server host to find out about error reporting. I will respond shortly.

I sent you an email. I found the error log but I was having a lot of other problems with the site, so I'm starting again.

I kept having this issue also - a blank front page, expecially under heavy usage. The error log in Cpanel told me that there were too many redirections on the index.php, Firefox even gave me the same issue, saying that it was determined that the index was redirected in such a way that it was not possible to resolve it. I found that if I deleted the installation directory everything came to life - no problems, and very fast. When I need to install new sites, I just re run the patches in Multisites, and create the sites I need, then I delete the installation folder again afterwards, just to prevent the issue later. It doesn't take long, and even with this work around JMS is a great program.

The "ping-pong" between the /index.php and the /installation/index.php is something that we have fixed in joomla 1.7 some time ago.
So if you had such problem with the "/installation" directory of joomla 1.7.
Checked that you used the latest JMS version.

See the FAQ for the procedure to get the latest version.