https://jitpack.io/com/github/jMonkeyEngine/jmonkeyengine/master-v3.1.0-beta1-g082fea9-708/build.log
It looks like something is broken in dependencies.
Probably not wrong, I also had this issue when building the sdk.
It looks like sourceforge is having server issues (I had 502 Broken Gateways) and it’s a Timeout, which is different from a 404.