If you send a template with just a software distribution task in it to a machine that is already imaged and functioning, does it work properly? If it does then it is likely that the device hasn't authenticated with the domain and thereby has no access to the share. You could use an HTTP share and solve that issue or join the domain, reboot, auto login as a domain user before distributing software. During a provisioning task the computer account is used to authenticate with the UNC share.