API_ERROR Reference update failed

I’ve been using Netlify CMS for many month now, and we never had any issues. But, since yesterday, editors of our website told us there were a problem.
When I go to Editorial Workflow, we get this message:
Netlify CMS is adding labels to 1 of your Editorial Workflow entries. The “Workflow” tab will be unavailable during this migration. You may use other areas of the CMS during this time. Note that closing the CMS will pause the migration.

and when they try to publish or save a post, they get Failed to persist entry: API_ERROR: Reference update failed.

On Github, I have 3 branches currently: ‘master’ (Default), ’ cms/articles/the-slug-of-the-post’ (Active) and ‘cms/podcast/name-of-a-podcast’ (Stale).

What can I do to resolve the situation?

This is usually happening when you’re using the editorial workflow but already have a branch called cms . If that’s the case, it can be fixed by removing the cms branch from the repo.