Failure while attaching runs to study on Test server

Description

The openml-python API has a unit test failing with a server exception. In the unit test, a study is being successfully uploaded, following which more runs retrieved from the server are being attached to this study. However, this fails with the error:

openml.exceptions.OpenMLServerException: https://test.openml.org/api/v1/xml/study/684/attach returned code 1045: Problem attaching entities. Please ensure to only attach entities that exist - None

Steps/Code to Reproduce

The POST request to the server which results in the above error should be reproducible using curl:

curl --data "api_key=610344db6388d9ba34f6db45a3cf71de" --data "ids=166,167,168,386,480,481,512,516,520,772" https://test.openml.org/api/v1/xml/study/685/attach

or Python:

import requests
r = requests.post(url="https://test.openml.org/api/v1/xml/study/685/attach", data={'ids': '166,167,168,386,480,481,512,516,520,772', 'api_key': 'c0c42819af31e706efe1f4b88c23c6c1'}, files=None)
print(r.status_code)
print(r.text)

Expected Results

Should return a response code of 200.

Start with the openml-python unit test that uploads study 685 and attaches runs, then reproduce the failure using the POST request to /api/v1/xml/study/685/attach shown in the issue. Trace the server-side handling of the attach endpoint and verify that the reported IDs can be attached successfully with an HTTP 200 response.