I have 7 blogs in one Network Install (or multisite for some of you). These are all hosted in one package on a Network solutions shared host. I recently learned that you can use their web control panel to point domains to your packages and/or subfolders.
I took advantage of this and in so-doing found that with my package I can only point three domains to folders in a package (5 and 10 respectively for the Large and Unlimited packages).
Directly after that I noticed that the folders which I had pointed domains to would give me a 403 Forbidden Error when I tried to access the site but not when I tried to access the wp-admin page.
The thing that caused this... is that when you point a domain to a sub-folder from the network solutions web page, and that folder doesn't exist (as is the case with the network installs' sub-folder structure) the folder will be created.
If you have an actual sub-folder with the same name as the network install's pseudo-subfolder (it's actually a mod_rewrite that gives the illusion of a subfolder in a network install) the adress http://example.com/examplesubfolder will point to the actual subfolder and not to the examplesubfolder site.
Most of the support that I was able to find gave me a solution but this is not a problem with mod_security.