In case anyone finds this thread seeking a solution, here's what worked for me- I wrote my host and told them about what I'd learned in this discussion. They wrote back with this reply:

"This is actually a security feature of suphp. I have gone ahead and disabled it by modifying your suphp.conf file to include the line:

check_vhost_docroot=false

As you can see, when this is enabled, you will not be able to execute PHP scripts external to their appropriate document roots. Please consider whether or not you actually want this disabled; if not, we can simply turn it back on for you and we'll find another solution for your needs."

So, it is both suPHP related, and indeed a permissions issue, but a simple fix.

Good luck, and I hope this saves someone a headache

Thank you for your contribution.

Hi,

I've purchased the multisite component last night and installed it on a J!1.6.5 site. Everything went well, until I wanted to access the admin site of the first slave site. I received an internal server error, and found out, that the service provider is indeed using suphp, as suggested in the previous message, but unfortunately they can't turn off the mentioned check_vhost_docroot to false, because of security issues.
Because of the service provider I must put the slave site to the same level in the directory tree as the master, I can't put it into a subfolder.

Do you have any workarounds for this problem?
I desperately need some help with this...

Thanks in advance,
Dave

Internal Server Error can be due to plenty of things and amoung them, the permission of the files and folders.

I suggest that you start to test the permission with the "hello world" to verify that youc an create symbolic link and directories and so on.

If the hello world is working and that you can also access PHP in a linked directory then perhaps you should also check if you have a "error_log" that would provide you more info.

Perhaps you have a log file that give more info on the reason of the "Internal Server Error".

Internal Server Error can also be due to htaccess that is not correctly setup.

The log says:
[Thu Jul 14 11:02:07 2011] [error] [client 85.90.163.10] SoftException in Application.cpp:221: File "/web/sites/mainsite/administrator/index.php" is not in document root of Vhost "/web/sites/slave"

I'm able to access php files within the "sites" directories, and from the error it's obvious the problem is with the "check_vhost_docroot" as artcop wrote.

Solution?

No - I don't see any solution if you do not accept to follow the symbolic links.

The only solution that I see is to not used the symbolic link and assign the other domain with the same directory as the master.
The limitation would be the "images", "templates" directories that will be more difficult to make specific for each slave sites.