[Windows] Submodule Checkout: Host key verification failed.

Summary

When trying to clone submodules from git on the Windows Multi-Runner v1.7.1, following error message is shown:

$ git submodule update --init --recursive

Submodule 'PROJ_lib' (git@sub.domain.at:abc/proj_lib.git) registered for path 'src/libproj'
Cloning into 'src/libPROJ'...
Host key verification failed.

fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
fatal: clone of 'git@sub.domain.at:abc/proj_lib.git' into submodule path 'src/libPROJ' failed

Used Software

• GitLab Community Edition 8.14.0 cde955c
• Runner Host: Windows 7
• Windows Multi-Runner v1.7.1
• git version 2.8.2.windows.1

Steps to reproduce

• Installed Windows Multi-Runner amd64 1.7.1 as a service
• Shell is set to cmd
• Used .gitlab-ci.yml as stated below

.gitlab-ci

stages:
  - build
  - doc
  
 before_script:
  - git submodule update --init --recursive
  
build:windows:
  stage: build
  tags:
    - windows
  script:
    - dir && "C:\Program Files (x86)\Microsoft Visual Studio 14.0\Common7\IDE\devenv.com" Project2.sln /Build "Debug|x86"
    - .\build\Debug_Win32-vc140\libProject2\test\test.exe
doc:windows:
  stage: doc
  tags: 
    - windows
  dependencies: 
    - build:windows
  script:
    - .\bin\doxygen.exe public.dox

Output

Running with gitlab-ci-multi-runner 1.7.1 (f896af7)
Using Shell executor...
Running on HOST001...

Fetching changes...

Removing Build/
Removing Debug/
Removing GoogleTest/Debug/
Removing Project2CLI/Debug/
Removing Visual Studio 2015/
Removing Visual Studio 2015Templates/
Removing log/
Removing src/libProject2/fortran/Fortran.u2d
Removing src/libProject2/scripts/input_definition.pyc
HEAD is now at d294948 CI
From http://sub.domain.at/abc/proj2
   d294948..1d69ef5  gitlab-ci  -> origin/gitlab-ci
Checking out 1d69ef57 as gitlab-ci...

$ git submodule update --init --recursive

Submodule 'PROJ_lib' (git@sub.domain.at:abc/PROJ_lib.git) registered for path 'src/libPROJ'
Cloning into 'src/libPROJ'...
Host key verification failed.

fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
fatal: clone of 'git@sub.domain.at:abc/PROJ_lib.git' into submodule path 'src/libPROJ' failed
ERROR: Build failed: exit status 128