I'm not sure if this would be the right place to report this, but it seems as downloading from your build server isn't working. This goes for trying to download any build from there.

Thanks for reporting! I had no idea...
It looks like + isn't being escaped in the links' URL's.

For now you can replace + with %2B in the URL to get the right download link:
https://s3.amazonaws.com/fastcraft/circleci/357/fastcraft-3.0.0-unknown%2Butc.20190521182502.jar

Should be fixed now :)

Thank you!