Errors Leaving Comments Some Users Only

Hi

I ahve lots of users who are not acutal ‘registered Users’ but anonymous postes who can post comments against each Wordpess post – now I hve had 4 people out of hundereds only with this problem.

Also it does not happen on all posts and not all the time, basically when they try to leave a comment they receive the following error:

You are not authorized to view this page
You might not have permission to view this directory or page using the credentials you supplied.

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If you believe you should be able to view this directory or page, please try to contact the Web site by using any e-mail address or phone number that may be listed on the website home page.

You can click Search to look for information on the Internet.

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Does anyone know what could be causing this.