Vibe.d HTTP Request strange behavior - Second attempt to send HTTP request failed

[Voxel]

We would like to use vibe.d for a simple backend, but run into 
strange behavior with HTTP request. The backend is simple: it 
receives requests from clients and makes requests for some json 
contents. Now, the code is only a vibe.d basic http server and a 
sample http request from vibe.d example.

Sometimes(very often) we got the error:

"statusMessage":"Second attempt to send HTTP request 
failed.","statusDebugMessage":"object.Exception at ../../../.dub/packages/vibe-d-0.8.6/vibe-d/http/vibe/http/client.d(615): Second attempt to send HTTP request failed.\n----------------\n/usr/include/dlang/dmd/std/exception.d:516 pure..."

It starts happening when there is no request for a few seconds. 
The error comes only, when I read the response (res.readJson(); 
or res.bodyReader.readAllUTF8();).

Our simple test case is: start vibe.d server, open a browser. 
Opening our server url makes a http request to our vibe.d server, 
which tries to make a http request to an another server.

I found some related content here: 
https://github.com/vibe-d/vibe.d/issues/2290
And here (second commenter is me): 
https://github.com/vibe-d/vibe.d/issues/2451

We use the latest stable vibe.d(0.8.6), dub and dmd.

We tried several things, but nothing works.