varnish 3 and FetchError http first read error: -1 0 (Success)

Wed Dec 16 09:05:06 CET 2015

On some (10%) post requsest i get status 503 (Service Unavailable).
I really can't figure out why that is happening.
And always when it happen I get additional FetchErrors:

Resource temporarily unavailable
backend write error: 11 (Resource temporarily unavailable)

How can I debug it, or what is the cause?
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