Seems to be a server issue going on.
SV downloads working fine here. And build systems are better than downloading the Sponge API jar.
Think it’s the same thing I’ve reported here.
The first page of downloads works fine for me, but it 503’s when trying to view older builds.
From this article it sounds like the first_byte_timeout setting is maybe a bit too keen? (not at-all familiar with Varnish cache software tbh)