1. Accessing network shares (UNC paths) is different than local folders in regards to user context.
By default, ASP.NET applications run under the context of NETWORK SERVICE or ASPNET users but when accessing a network resource, the other machine will not recognize those users as they are local users. So you need to run the application under the context of a common user which you can add to the ACL of the network share with the required permissions.
If there is a domain, you can use a domain user for this purpose. When you include the below settings in web.config:
<authentication mode="Windows" />
<identity impersonate="true" userName="SomeUserX" password="Password123" />
FileVista will be executed directly under the identity of SomeUserX and not under system accounts like NETWORK SERVICE or ASPNET. So, you need to make sure SomeUserX can access the share. If you are connected to a domain, this would be easy because you would go to the other machine and give permissions directly to this domain user as you can select him in the list.
However, you can not select and add a user of another computer if there is no domain. In this case you need to create a user with same name (eg. SomeUserX) and password on the other server and use this common user for the impersonation setting above. FileVista is dependant on Windows permissions so you should first try in Windows if you can access the share with the user SomeUserX that is created on both machines. If Windows can access then FileVista will be able to access too.
2. Drive letter mapping is not possible as it is an option only for the interactively logged in users and it’s not available to ASP.NET applications like FileVista.
3. FileVista will not count the number of users in your AD database. I guess you opened the FileVista database (mdb file) and tried to add a new record (6th user) in the User table manually. Even if you delete this extra record, you need to restart FileVista so that it can recount the actual users in the table and recover from error state. You can restart FileVista by touching (opening and saving) the web.config file. Note that if you need AD integration, you will need to create corresponding users in FileVista for each AD user. This means, if 100 AD users will log into FileVista, then you will need at least 100 users license for FileVista.
4. The showing of FileVista's own login page is dependant on the IIS's authentication settings. If you turn off Anonymous authentication, then browser's own authentication dialog will be displayed instead of FileVista's login page. So when you turn off Anonymous authentication and add these line to web.config:
<authentication mode="Windows" />
<identity impersonate="true" />
you will be authenticated against AD and FileVista will run under the context of the credentials you supply. This means FileVista will be limited by the NTFS permissions of this AD user.
So when you uninstalled and installed FileVista, I guess the only difference was the IIS's authentication setting. You also got rid of the "users exceeded" error message because you started with the new unmodified database.