Syncing to GitHub suddenly failing

I’ve been using Pontoon for an open-source project for over 6 months, but suddenly syncing to GitHub started failing:

Failed to sync locale de for project <myproject> due to commit error: error: src refspec i18n does not match any.

This still worked a week ago. i18n is the branch which I use for translation syncs. It still exists in GitHub so not sure why it’s complaining about it. Any idea what could be going wrong or how I could start investigating the problem?