SVN commit not working

I'm having a problem committing changes from QtCreator. Here's what's happening:

1. I have a modified file in a project hosted in an SVN repo.
2. I press Alt+S, Alt+C (SVN commit hotkeys) an window opens in order to type the commit message.
3. I type "Nonessential." (without quotes) and press Ctrl+Enter to perform the commit.
4. QtCreator displays an error window (screenshot attached). Even if I press Yes, the commit doesn't happen, but...
5. ...If I try to perform the commit again, the Version Control panel at the bottom of the screen appears saying "Another commit is currently being executed."

The same error appears if I just try closing the commit window, without actually doing the commit (Ctrl+W).

I don't have any commit hooks in this repo and performing the same commit by external tools succeeds.