i guess that the problem is that QlikView has been installed with a local account. the QLikView services starts with this account. and this account can't reach the folder where the excel is, it has not the permissions to access it.
As Daniel suggested, you can map the folder using an account with permissions to access the folder.
Another work-around could be to create a local account on the file-server (where you host the xls-file) with the exact same name and password as the local account running the services on QVS. As you're using local accounts, Windows will use NTLM as the authentication protocol between the resources, and with the exact same name and password, the hash generated will be the same on the two machines. This is a way to share resources in a Workgroup, where no domain controller is present.