Giving IIS access to files shared from a mac

I'm relatively new when it comes to network administration, but I have put in many hours of research and effort before asking this question. I know my question is similar to many others, but I don't believe it is a duplicate. My apologies if I'm mistaken.

Basic setup/problem:

I'm trying to run a PHP/Codeigniter website with IIS on a Windows virtual machine, but with the source code shared from the host (which is a mac). I'm getting the following IIS error when I try to load the home page:

Details on error:

• 500.19 - The requested page cannot be accessed because the related configuration data for the page is invalid
• Config Error - Cannot read configuration file (web.config)
• Error code - 0x8007010b

According to this page, "This problem can occur if the specified content directory cannot be accessed." https://support.microsoft.com/en-us/help/942055/http-error-500-19-error-when-you-open-an-iis-7-0-webpage

The crux:

So apparently I haven't successfully given IIS permission to the files on the mac side.

Some details:

• The host is a macbook running Catalina 10.15.
• The host is sharing files via System Preferences -> Sharing.
• With VirtualBox 6.0.14, I'm running a guest OS, which is Windows v1909.
• The vm is attached to Bridged Adapter.
• IIS version is 10.0.18362.1
• The web.config file is already there, and it runs fine on other deployments of this site (that aren't trying to used shared files), so it's fine.

Here are things I have in place, which I've learned from many similar questions:

• The mac user I'm using has "Read & Write" access. I can access, edit, and save the shared files in Explorer on the Windows VM.
• The windows user I'm using has the exact same name and password as the mac user
• The windows user is an admin
• The Application Pool is configured to use that Windows user as the Identity (under Process Model)
• The site's Authentication is set to use that Application Pool identity
• In IIS -> the website -> Basic Settings -> "Test Settings...", Authentication and Authorization are both passing the test. It says, "The specified user credentials are valid."
• The URL Rewrite extension for IIS is installed
• No impersonation flag is set in php.ini.

As mentioned above, the site I'm trying to run is built on PHP/Codeigniter. That's where I'm getting the error. However, if I change the "Basic settings" of the website to point to a simple html file shared from the mac, it displays just fine. So I suspect the error may have something to do with php, or with something more sophisticated than just displaying an html file.

Any help would be very much appreciated!