Shell runner cannot upload artifacts: "501 Not Implemented"

Hello!
I’m having trouble using job artifacts with Gitlab CI.

I’m working on dockerized web app. I run the app with docker-compose and run some tests on it.
When the tests fail, the test framework takes a few screenshots. I then copy the screenshots to the build directory, and try to upload them as artifacts, but I get the following:

Uploading artifacts...
Runtime platform                                    arch=amd64 os=linux pid=6560 revision=d0b76032 version=12.0.2
src/tests/_output/: found 5 matching files         
WARNING: Uploading artifacts to coordinator... failed  id=11975 responseStatus=501 Not Implemented status=501 Not Implemented token=EyreCNs7
WARNING: Retrying...                                error=invalid argument
WARNING: Uploading artifacts to coordinator... failed  id=11975 responseStatus=501 Not Implemented status=501 Not Implemented token=EyreCNs7
WARNING: Retrying...                                error=invalid argument
WARNING: Uploading artifacts to coordinator... failed  id=11975 responseStatus=501 Not Implemented status=501 Not Implemented token=EyreCNs7
FATAL: invalid argument                            
ERROR: Job failed: exit status 1

Here’s the stage configuration:

upload_screenshots:
  stage: upload_screenshots
  script:
    - docker cp myapp_php_test:/var/www/html/myapp/tests/_output/. src/tests/_output/
  when: on_failure
  artifacts:
    paths:
      - src/tests/_output/
    expire_in: 1 week

I made a test project that simply creates a text file and uploads it as an artifact, and it works as expected:

test:
  script: 
    - echo "test" > test.txt
    - ls -la
  artifacts:
    paths:
    - test.txt
    expire_in: 1 week
$ echo "test" > test.txt
Uploading artifacts...
Runtime platform                                    arch=amd64 os=linux pid=101330 revision=d0b76032 version=12.0.2
test.txt: found 1 matching files                   
Uploading artifacts to coordinator... ok            id=11946 responseStatus=201 Created token=xmkvqina
Job succeeded

I’m running Gitlab CE 11.3.5 (c74e8d9) and Gitlab Runner 12.0.2 (d0b76032). The respective servers are not on the same subnetwork. I thought it might be our proxy but I don’t know how to make sure of that. Besides, my test project works.

How can I debug this? Is there a way use the artifact-uploader verbose mode from the gitlab-ci configuration?

It was the freaking proxy :sweat: After setting the no_proxy environment variable in the CI settings to our internal subdomain, artifact uploading works as expected.