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Thank you for your question. Without being able to duplicate it, we can only take a guess, but here are some suggestions.
Is the user you are using allowed access to this area?
Is this an article that was made public in the back end?
Have you cleared your browser cache?
Have you adjusted the Search Engine Friendly URLs in the back end?
What specifically did you do from the last time it worked until the first time you got the error?
I would suggest checking out this post from the Joomla Support Forums in regards to the error type that you're getting. It may help your problem.
You might want to check and review @haticeguney's idea. This might help resolve your problem.
In addition, we sometimes recommend embedding the form using its iframe embed code particularly on CMS with complex scripts. This is to prevent any script conflicts that would change the form behavior or perhaps break its functionality. Please try it and let us know here if none of the above suggestions helped you.
@Alan, I think I have misunderstood your question. @haticeguney is right. That usually happens when you do not have access permission to the page or perhaps the page wasn't fully published yet that's why it is yielding that error when you are viewing it while not logged in. Kindly check.
Also, can you please provide the exact URL of the page where you are getting that error? It would be 10 times easier to troubleshoot the problem if we can inspect it on our browser.