Network error when trying to access website files

I have a website using anonymous authentication with files on a network share and an Active Directory service account configured for "Connect As". 

The website works, but when I try to open the default website in IIS Manager, instead of showing the files it throws an error message "Network error". Same issue when I try to create a Virtual Folder.

I thought it may have been an account issue, but using this account on another IIS instance in exactly the same way, with the same physical path, works as expected.

Kind of stuck, don't know where to go from here.

Thanks in advance!