The issue that was causing problems when using HTML tags in comments has now been resolved so please ignore my original post (see below) and continue to use them if/when required.
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Original post:
During the last few days a problem has arisen when using HTML tags (eg for bold or italics) in comments. If used, when the comment is submitted an HTTP403 error page is displayed and the comment is not posted.
It is not immediately apparent as to the cause of this problem since there are no relevant entries in any of the error logs and also I have been able to determine that it is not due to the WordPress update earlier this month nor to some more recent plug-in updates.
I will continue to pursue this issue but until I get it resolved please do not try to use any of the HTML tags in your comments.