The only other thing I can think of is that trying to use a native app with a Dockerised app is causing an issue. That’s what the volume mapping tries to work around.
Uninstall NZBGet (or maybe just disable in App Central for now) and install it using Portainer instead - https://hub.docker.com/r/linuxserver/nzbget/
If you need specific instructions of how to do so, let me know (I know that working it out the first time was a pain for me).
Promise this is my last stab at this, I’m out of ideas now.