Varnish returning synthetic 500 error even though it has stale content it should serve. But only seems to happen during/after a burst of traffic

[Batanun B]

> I'd suggest to determine the correct delivery of the front page via an external monitoring
> (e.g. icinga2 or a simple script). As far as I understand, you don't need to
> know the exact request, but more of a rough point in time of when the requests
> start failing. So a monitoring script which curls every minute should be
> sufficient and causes a lot less trouble.

Well, in our use case we would need the exact requests in order to understand why it happens. It might very well be a bug in our VCL, but I would like to have show me something that could narrow it down.