The macOS slave is broken since two days for a very stupid reason: sourceforge is sluggish and provides downloads at 50kB/s, so the big boost tarball always fails to download.
The official Boost website does not propose source forge download links, so I used the official link instead (downloads in a few seconds).
The only remaining use of sourceforge is for libpng, it is still the official download link, so I don't touch it.