mshots "file not found" ?

I’m using the magnificent WordPress mshots and plugin courtesy of WordPress.com and Binary Moon respectively. I’m using it on self-hosted, and it’s sensational. However for one particular URL, a “file not found” error is showing. The URL is certainly correct – has anyone had a simular issue? Or any suggestions?

I did read on the Binary Moon that the rendered screenshots are cached, and refreshed every few hours. I’m hoping a flush might fix the problem, but it hasn’t yet.