Unable to reply to post

Hi, John was helping me out on a recent post however I am no longer able to reply... I can preview the message fine, but whenever I try and then post ("save") it I keep getting an error saying: "The website encountered an unexpected error. Please try again later."

I've tried posting a markdown, html and plain text, and replying to both his comment and to the post overall but no joy -- Is anyone able to advise please?

Thanks, Martin