given the error this is something that is created by the varnish instance that is in front of the buildfarm. On a quick look I could immediately figure out what the problem is - but it looks like you (or somebody else) tried at least to click one of the links above using hist desktop browser and got an error about a missing branch specification ;)
AFAICT:
A quick look in the logs indicates that the buildfarm is responding:
- RespHeader Status: 492 bad branch parameter
However, 492 is not a valid http status code, so Varnish can't handle it and thus returns 503 failure to the client.