IIS Website displaying server error in '/' Application

What are you doing exactly? Are you using some other custom made package?
Most probably, it's just not designed to work from a remote server. Ask the developer of that custom package you're using.

I would agree with Kim but try using 2 slashes instead of one.

eg:  \\\\<ip>\\path

The other thing you can try is mapping that drive to say X: and then use that as a local drive.

The IIS user does not have permission to access other servers.

What permission is needed

Try the 'mapped drive' approach.  I don't think you are able to give the IIS user permission to access other servers.

Mapped drive didn't work

please assign the Network services and IIS_Users permission, IIS_WPG group

Are these permissions assigned on the it's server
Or the server that is hosting the file share

It's the permissions of the IIS User.  It is a special user for the web server and it is intentionally limited by design.

Yes but what server
I have the fronet end server with its accessing a file share on another server that does not have its
Where do I configure permission

In all the previous questions about accessing a remote server from IIS, the answer has always been that you can't.  The problem has always been that the IIS user does not have and can not be given permission to access other machines.

I have 2 iis servers that are on a subnet
if I try the storageroot0 location as local on one of the iis servers it works and even if I have the folder on the second iis server as a unc path it works
but if I have the file location on another server that does not have iis installed and on a different subnet I get the error

If that's the case, it's still an authentication problem, and if different subnet, check also if your network settings are correct (source should be able to ping destination and vice versa).

I can ping and access the file via cifs or explorer
its just browsing the website from the iis server fails

that or map the drive as a persistent drive.

just create the same local account?
or maybe install  the iis role on the file server
that would install the iis accounts

You're better off switching to a domain account for your application pool, then granting permissions on the file server for that account.

Note you should add the same domain account to the IIS_Users local group on the web server.  The IIS user is a local account, so even if you were to install IIS on the file server, it wouldn't grant permissions.

the 2 iis servers will be out on the dmz
the file share will be inside the lan