conan: Bintray 404 when connecting from IPv6

If I run

$ conan install -s build_type=Release -s compiler=clang -s compiler.version=3.8 -s compiler.libcxx=libstdc++11 --build=missing

I immediately get the output

Catch/1.9.6@bincrafters/stable: Not found in local cache, looking in remotes...
Catch/1.9.6@bincrafters/stable: Trying with 'conan-center'...

but then it takes 4 minutes till

Downloading conanmanifest.txt

and then another 8 minutes till

[==================================================] 58B/58B          
Downloading conanfile.py
[==================================================] 866B/866B        
...

Why does it take that long? How can I speed it up? I have this issue with all packages that I tried.

---

$ conan --version  
Conan version 0.28.1
$ conan remote list
conan-center: https://conan.bintray.com [Verify SSL: True]
conan-transit: https://conan-transit.bintray.com [Verify SSL: True]