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.

The web site has been updated, all the download links now refer to 
https://boostorg.jfrog.io/artifactory/main/release/

Example:
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
(The pre-1.63 releases are still hosted at SourceForge)

— Marshall