howto resolve 10.in-addr.arpa

[A. Schulze via Unbound-users]

Hello,
we have the following configuration to point unbound-1.5.4 to our  
private nameservers:

server:
  local-zone: "10.in-addr.arpa." transparent
  domain-insecure: "10.in-addr.arpa."

stub-zone:
  name: "private.example.com."
  stub-addr: "10.0.1.53"
  stub-addr: "10.0.2.53"

stub-zone:
  name: "10.in-addr.arpa."
  stub-addr: "10.0.1.53"
  stub-addr: "10.0.2.53"

----

this setup run here for years.
Now the server 10.0.2.53 died and we notice timeouts for  
*.10.in-addr.arpa PTR queries.
But we did /not/ notice any problems for *.private.example.com.

In the last hour I also found that "transparent" is not the best declaration.
I may be better to configure ` local-zone: "10.in-addr.arpa." nodefault `
But as I don't have any local-data there should be no difference  
between nodefault and transparent.

I did not expect that unbound has trouble it one of two nameservers  
for a zone fail.
Any suggestions?

Thanks
Andreas