Plex offline Download option never working

Plex Media Server NAS & Devices
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5

I think it is dns rebinding protection issue - possibly same issue as Server crashing hard when trying to download - #12 by sa2000

This is failing:
https://192-168-178-54.c35706253ccb4dca856a51903b66eb4c.plex.direct:32400/library/parts/122297/1575798134/file.flac?download=1&X-Plex-Token=...c"}} 

01-03 13:51:57.709  e: [Downloader] Exception downloading /data/user/0/com.plexapp.android/Plex Media Server/Sync/1/18/.grab/d6d01c62422aaf7b152a26a4ef3ef8a1a2b70d86-local/02 - Rag Doll.flac

01-03 13:51:57.710  i: [DownloadQueueManager] Download failed: d6d01c62422aaf7b152a26a4ef3ef8a1a2b70d86

01-03 13:51:57.710  e: An exception occurred: java.io.IOException: Unsuccessful response: 0
01-03 13:51:57.710  e: Stacktrace: java.io.IOException: Unsuccessful response: 0
	at com.plexapp.downloads.m.d(SourceFile:275)
	at com.plexapp.downloads.m.b(Unknown Source:0)
	at com.plexapp.downloads.m$a.invokeSuspend(SourceFile:13)
	at kotlin.coroutines.jvm.internal.a.resumeWith(SourceFile:12)
	at kotlinx.coroutines.b1.run(SourceFile:119)
	at kotlinx.coroutines.internal.n.run(SourceFile:13)
	at kotlinx.coroutines.scheduling.k.run(SourceFile:3)
	at kotlinx.coroutines.scheduling.a.k(SourceFile:1)
	at kotlinx.coroutines.scheduling.a$c.d(SourceFile:15)
	at kotlinx.coroutines.scheduling.a$c.n(SourceFile:29)
	at kotlinx.coroutines.scheduling.a$c.run(Unknown Source:0)

01-03 13:51:58.040  i: [SourceManagerStorage] Correctly saved 15 sources (9 pinned) (15 previously pinned).
01-03 13:52:00.712  i: [DownloadQueueManager] Network conditions are fine, download really failed

01-03 13:52:00.713  w: [DownloadQueueManager] Adding d6d01c62422aaf7b152a26a4ef3ef8a1a2b70d86 to the failed list: Unknown issue occurred.

Let me know if making the changes to the FritzBox configuration makes a difference