Docker image upload to own registry blocked?

Is the upload to a own/self hosted docker registry blocked by github?

Downloading images from our registry works just fine but if we try to push images we get the following error message after around 17 minutes:

Error: buildx call failed with: error: failed to solve: rpc error: code = Unknown desc = failed commit on ref "config-sha256:xxx": failed to do request: Put https://registry.myurl.com:443/v2/project/server/blobs/uploads/xxx?_state=xxx: read tcp 172.xxx.xxx.xxx:44340->185.128.xxx.xxx:443: read: connection timed out

Our registry hoster could not see any blocked packages/requests.

This is the step that fails (a docker login that is done before works just fine):

      - name: Docker | Build & Push
        id: docker_build
        uses: docker/build-push-action@v2
        with:
          context: ./server
          file: ./server/Dockerfile
          tags: |
            ${{ env.DOCKER_REGISTRY }}/project/server:latest
            ${{ env.DOCKER_REGISTRY }}/project/server:${{ env.VERSION }}
          push: true

This issue was related to the network of the hoster. The mtu size was configured to high so that packages/fragments got lost. We did not have the issue when pushing from somwhere else. (most likely because the mtu was configured to a safe value there and github uses a different value per default)

Our hoster configured a safe mtu size for the runtime what solved the issue.