Empty error message on trying to add another user's submission to leaderboard

If you are having trouble using the site:

I host a competition on our own instance of codabench (latest commit: 03111359). The leaderboard uses the rule "Force Best".

I open the submissions overview on the competition page while logged in to the competition organizer's account. Then I click the "Add to Leaderboard" button for a submission by the organizer and a submission by a normal participant.

Expected

Because of the chosen leaderboard setting neither of the two submissions can be added to the leaderboard manually and both times an error should be shown in a red box in the top right corner.

Actual

Because of the chosen leaderboard setting neither of the two submissions can be added to the leaderboard manually. When I try to add the submission by the organizer the error is shown. When I try to add another user's submission the error message is empty and an empty red box is shown.

The request to http://.../api/submissions/ID/submission_leaderboard_connection/ returns json {"detail":"Not found."}