Blank Error Message when trying to add photo from story form

I can't work out what's causing this problem:

I am trying to add the attached image from the story posting form at http://www.mysite.com / linkremovednow

When I do so I get a red error box - but it has no text inside it to explain what the error is.

The strange thing is I was successful in adding the exact same image to the timeline of http://www.mysite.com/ linkremovednow

That's in an event in exactly the same category

The only difference I see is one has a URL from being a menu item, whereas the other is a standard ES generated URL.

Can you work out what the problem is, and why that error message is blank?

Thanks. It is a live page with lots of attendees on this event who get email notifications - so hoping any testing won't require multiple postings!