SSIS package from ASP.net does not work on client but does on server - iis

Question

Description Of Environment
I have an SSIS task which executes from asp.net, the package gets data from excel files over UNC path and puts the data into a sql server databaase . I have deployed the SSIS to file system, it has windows authentication database connections and the IIS user has database access. I can log in as the AppPoolUser I have used to host the web application and open/modify the files in question so those basic permissions are there. Web App is compiled in x86.
When it works:
When run from web app in browser on server under my user or appPool user
When logged in as me on server running through BIDS
When logged in as AppPoolUser on server running through BIDS
When run from scheduled task on server using AppPoolUser
When it doesn't work:
When run from client browser i.e. not the machine that has IIS hosting the asp.net app
My Question
What is different about the client and server and how do I make it work? I was under the impression that when running the web app all connections go through the AppPool User so it should behave the same on any machine server included?
The Error returned to client browser
SSIS Error Code DTS_E_OLEDBERROR. An OLE DB error has occurred. Error code: 0x80004005. An OLE DB record is available. Source: "Microsoft Office Access Database Engine" Hresult: 0x80004005 Description: "The Microsoft Office Access database engine cannot open or write to the file '\\myserver\My folder\myfile.xlsx'. It is already opened exclusively by another user, or you need permission to view and write its data.". SSIS Error Code DTS_E_CANNOTACQUIRECONNECTIONFROMCONNECTIONMANAGER. The AcquireConnection method call to the connection manager "MyExcelFileConnection" failed with error code 0xC0202009. There may be error messages posted before this with more information on why the AcquireConnection method call failed. component "Excel Source" (1) failed validation and returned error code 0xC020801C. One or more component failed validation. There were errors during task validation.
c# Code running SSIS Package
string pkgLocation;
Package pkg;
Application app;
DTSExecResult pkgResults;
string result = string.Empty;
string dtsErrors = string.Empty;
try
{
pkgLocation = packagePath;
app = new Application();
pkg = app.LoadPackage(pkgLocation, null);
pkgResults = pkg.Execute();
foreach (DtsError local_DtsError in pkg.Errors)
dtsErrors += " " + local_DtsError.Description;
result = pkgResults.ToString();
}
catch (Exception exception)
{
result = exception.Message;
}