The structural, electronic and ion transport properties of an amorphous member of the LiPON family with non-trivial composition and cross-linking are studied by means of electronic structure calculations within Density Functional Theory. By a combination of an evolutionary algorithm followed by simulated annealing and alternatively by a rapid quenching protocol, structural models of disordered Li5P4O8N3 are generated, which are characterized by a local demixing in Li-rich and Li-poor layers. These structures have a composition close to what is found experimentally in thin films and contain all the expected diversely coordinated atoms, namely triply- and doubly-coordinated nitrogens and bridging and non-bridging oxygens. The issue of ionic conductivity is addressed by calculating defect formation energies and migration barriers of neutral and charged point defects. Li+ interstitials are energetically much preferred over vacancies, both when the lithium reservoir is metallic lithium and LiCoO2. The competitive formation of neutral Li interstitials when LiPON is contacted with metallic Li results in the chemical reduction of LiPON and the disruption of the network, as recently observed in experiments.