As Mistral mentioned that backup images are intended to store in the same subnet where these has been taken and can not be replicated in between Site Server or NS. Also backup images can be imported by mentioned tool but are un-syspreped and must be used for the same hardware on which it’s been created.
But I would like to understand your issue bit more; please let me know if I am getting this statement “When the back-up proceed sometimes the destination path points to PXE, sometimes points to Altiris Server.” is correct where:
1. PXE/Site Server is in different subnet and not installed on NS
2. NS is not a Package Server
2. Issue is where “for same backup task and same client” is pointing to either a Site Server (Where PXE server is) or to NS/Altiris Server and connection is not consistent.