Blank composer screen when creating or editing a blog post from the front-end

All of a sudden yesterday, after publishing the site (which up to that point was in development and content was only available to registered users), the composer appears to have stopped working. When a registered user logs in at the front-end and attempts to create or edit a blog post, the composer window opens and is blank with only the red corner cross showing to close the window.

The composer opens fine from the back-end.

I have seen similar issues in the forum and have checked that there is a default category, which there is. I've also checked ACL settings for registered users and these seem to be as they should.

I'm hoping you can help fix this for me.

Thanks

Paul