Thank you!
Definitely, FS uses short DNS name and it wasn't written to /etc/hosts. I've fixed it, but now get more complicated 500 error.
I'm using all-in-one server for tests, so I dumped 24517 port where FS sends mp3 file.
root@kzdev:/home/admin/# ss -antp|grep 24517
LISTEN 0 1000 *:24517 *:* users:(("beam.smp",pid=10256,fd=27))
T 10.2.0.237:24517 -> 10.2.0.237:12388 [AP]
HTTP/1.1 500 Internal Server Error.
server: Cowboy.
date: Tue, 17 Oct 2017 16:53:36 GMT.
content-length: 304.
.
{socket_error,
{nxdomain,
[{lhttpc_client,send_request,1,
[{file,"src/lhttpc_client.erl"},{line,222}]},
{lhttpc_client,execute,9,[{file,"src/lhttpc_client.erl"},{line,171}]},
{lhttpc_client,request,9,
[{file,"src/lhttpc_client.erl"},{line,93}]}]}}
No idea which hostname the Cowboy tries to resolve, but I checked the short name, the fdqn and the hostname of S3 endpoint - and they are resolvable. Unfortunately, there is nothing in the main log about this 500 error.