Bintray is “sunsetting” on the 1st of May. 
At that time, you will be unable to download boost releases from there.
Before that time, their service will be occasionally interrupted.


We will be updating the web site before then, but the new location is:

https://boostorg.jfrog.io/artifactory/main/release/

So instead of downloading from:
https://dl.bintray.com/boostorg/release/1.76.0/source/boost_1_76_0.tar.gz

You should download from:
https://boostorg.jfrog.io/artifactory/main/release/1.76.0/source/boost_1_76_0.tar.gz

This applies to all releases from 1.63.0 onward

— Marshall