Instalog doesn't use HTTP GET method, so I don't think this is caused by Instalog plugin.
Where can I find/reproduce this error message?

1. run py/shopfloor/factory_server.py
2. set toolkit to use shopfloor service url to the port running factory_server
3. watch output of factory_server.py . it will appear for ever minute.

I tried to kill Instalog instance on DUT and then it doesn't show the message any more, so it may be Instalog...

Sorry about that, Instalog will use GET method.
The output HTTP plugin will check the connection by GET method every minute, and legacy factory_server don't have input HTTP plugin. 
Maybe it is because the port using by HTTP plugins is same as the port using by legacy factory_server?

actually it's not a problem - the legacy factory_server does not support instalog, and we probably won't add instalog support to it.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform/factory_installer/+/a81834df008552e159a606532b920fd1a5f3b0bb

commit a81834df008552e159a606532b920fd1a5f3b0bb
Author: Hung-Te Lin <hungte@chromium.org>
Date: Fri Aug 25 12:34:11 2017

factory_install: Change 'hwid' to be optional component.

The 'hwid' is usually downloaded from factory server so we probably
don't need to have that as necessary in the factory bundle.

BUG= chromium:758457 
TEST=None

Change-Id: I17373f258621576c3d249cf12f4576fc2e6d6a26
Reviewed-on: https://chromium-review.googlesource.com/634963
Commit-Ready: Hung-Te Lin <hungte@chromium.org>
Tested-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-by: Ting Shen <phoenixshen@chromium.org>

[modify] https://crrev.com/a81834df008552e159a606532b920fd1a5f3b0bb/factory_install.sh

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform/factory/+/61446c21d0a23a2d8ceef98e0e5487dfe1b2dbf4

commit 61446c21d0a23a2d8ceef98e0e5487dfe1b2dbf4
Author: Ting Shen <phoenixshen@google.com>
Date: Fri Aug 25 12:34:11 2017

finalize_bundle: support unified build generated fw updater

1. Add optional 'project' argument, assume project=board for backward
compatibility.
2. After extract fw files, also search images in model/{project}/*.bin.
3. Remove undocumented 'add-files' argument.

BUG= chromium:758457 
TEST=Run finalize_bundle with board=project=coral, image version=
     1. 9868.0.0 (unified build enabled)
     2. 9870.0.0 (no unified build)

Change-Id: I6c985f091cf7a879fc5aaa1ac3c53a83da3b64bc
Reviewed-on: https://chromium-review.googlesource.com/631759
Commit-Ready: Ting Shen <phoenixshen@chromium.org>
Tested-by: Ting Shen <phoenixshen@chromium.org>
Reviewed-by: Youcheng Syu <youcheng@chromium.org>

[modify] https://crrev.com/61446c21d0a23a2d8ceef98e0e5487dfe1b2dbf4/py/tools/finalize_bundle.py
[modify] https://crrev.com/61446c21d0a23a2d8ceef98e0e5487dfe1b2dbf4/setup/BUNDLE.md

There is no legacy factory_server now.
This issue is obsolete.