Dependency Proxy always fails to pull large image (unityci/editor:2020.1.15f1-base-0.9)

I’m setting up CI for a new project, and thought I would use the dependency proxy to cache the docker image used in my pipeline as it is quite large (several GBs). However, any attempt to pull the image from the proxy fails. If I put the image link using the proxy into .gitlab-ci.yml, I always get this:

Running with gitlab-runner 13.9.0-rc2 (69c049fd)
  on docker-auto-scale-com 9538b0ab
  feature flags: FF_GITLAB_REGISTRY_HELPER_IMAGE:true
Preparing the "docker+machine" executor
Using Docker executor with image gitlab.com:443/chez-plantes/dependency_proxy/containers/unityci/editor:2020.1.15f1-base-0.9 ...
Authenticating with credentials from job payload (GitLab Registry)
Pulling docker image gitlab.com:443/chez-plantes/dependency_proxy/containers/unityci/editor:2020.1.15f1-base-0.9 ...
WARNING: Failed to pull image with policy "always": received unexpected HTTP status: 500 Internal Server Error (manager.go:205:928s)
ERROR: Preparation failed: failed to pull image "gitlab.com:443/chez-plantes/dependency_proxy/containers/unityci/editor:2020.1.15f1-base-0.9" with specified policies [always]: received unexpected HTTP status: 500 Internal Server Error (manager.go:205:928s)
Will be retried in 3s ...
Using Docker executor with image gitlab.com:443/chez-plantes/dependency_proxy/containers/unityci/editor:2020.1.15f1-base-0.9 ...
Authenticating with credentials from job payload (GitLab Registry)
Pulling docker image gitlab.com:443/chez-plantes/dependency_proxy/containers/unityci/editor:2020.1.15f1-base-0.9 ...
WARNING: Failed to pull image with policy "always": received unexpected HTTP status: 500 Internal Server Error (manager.go:205:900s)
ERROR: Preparation failed: failed to pull image "gitlab.com:443/chez-plantes/dependency_proxy/containers/unityci/editor:2020.1.15f1-base-0.9" with specified policies [always]: received unexpected HTTP status: 500 Internal Server Error (manager.go:205:900s)
Will be retried in 3s ...
Using Docker executor with image gitlab.com:443/chez-plantes/dependency_proxy/containers/unityci/editor:2020.1.15f1-base-0.9 ...
Authenticating with credentials from job payload (GitLab Registry)
Pulling docker image gitlab.com:443/chez-plantes/dependency_proxy/containers/unityci/editor:2020.1.15f1-base-0.9 ...

It loops forever. If I try to pull the image locally after using docker login to log into my dependency proxy, I get the following:

PS C:\Users\Lou> docker pull gitlab.com/chez-plantes/dependency_proxy/containers/unityci/editor:2020.1.15f1-base-0.9
2020.1.15f1-base-0.9: Pulling from chez-plantes/dependency_proxy/containers/unityci/editor
d519e2592276: Pull complete
d22d2dfcfa9c: Pull complete
b3afe92c540b: Pull complete
6b8d5fc726a3: Pull complete
2d52acceb2ad: Pull complete
62488afa0c6a: Pull complete
bd9371502f15: Pull complete
da3c0ac63128: Downloading
b14a205340fb: Download complete
fe661294a4fa: Download complete
received unexpected HTTP status: 500 Internal Server Error

Layer da3c0ac63128 (which is 2.5 GB in size) never finishes downloading. I have confirmed that I can successfully pull the node:alpine image using the dependency proxy either locally or on a gitlab runner, which makes me think the issue has something to do with this particular image.

I am using Gitlab.com with the free shared runners. Locally I am using docker version 19.03.13.

Any help would be much appreciated!