MLFLOW Artifacts stored on ftp server but not showing in ui

I use MLFLOW to store some parameters and metrics during training on a remote tracking server. Now I tried to also add a .png file as an artifact, but since the MLFLOW server is running remotely I store the file on a ftp server. I gave the ftp server address and path to MLFLOW by:

mlflow server --backend-store-uri sqlite:///mlflow.sqlite --default-artifact-root ftp://user:password@1.2.3.4/artifacts/ --host 0.0.0.0 &

Now I train a network and store the artifact by running:

mlflow.set_tracking_uri(remote_server_uri)
mlflow.set_experiment("default")
mlflow.pytorch.autolog()

with mlflow.start_run():
    mlflow.log_params(flow_params)
    trainer.fit(model)
    trainer.test()
    mlflow.log_artifact("confusion_matrix.png")
mlflow.end_run()

I save the .png file locally and then log it with mlflow.log_artifact("confusion_matrix.png") to the ftp server in the right folder corresponding to the experiment. Everything works so far, only that the artifact does not show up in the mlflow ui online. The logged parameters and metrics show up normally. The artifact panel stays empty and only shows

No Artifacts Recorded
Use the log artifact APIs to store file outputs from MLflow runs.

I found similar threads, but only of users having the same problem on local mlflow storages. Unfortunately, I could not apply these fixes to my problem. Somebody has an idea how to fix this?

from MLFLOW Artifacts stored on ftp server but not showing in ui