To clarify, are you able to reliably reproduce this issue every time you try? It appears this occurred due to a network issue, so I wouldn't expect that to re-occur each time. (If it does though, let us know as that may indicate a bug in the SDK.)
Anyway, it looks like a network timeout caused the client and server to get out of sync. The batchUploadFiles method doesn't offer a way to recover from that (or do so automatically), so you'd need to retry the upload entirely (e.g., using batchUploadFiles again) for the failed file(s).