I’ve been seeing this issue for quite some time and they are required to debug the output of a build/test.
Resolving circle-artifacts.com (circle-artifacts.com)… 52.6.227.217, 52.5.120.214, 54.85.71.97
Connecting to circle-artifacts.com (circle-artifacts.com)|52.6.227.217|:443… connected.
HTTP request sent, awaiting response… 503 Service Unavailable: Back-end server is at capacity
2016-04-15 11:30:12 ERROR 503: Service Unavailable: Back-end server is at capacity.
update: seems to be OK again