Well, I found that the problem here lies with the Quick Tabs plugin…which generates its own shortcodes INCORRECTLY. The generated shortcode began [quick-tab…] when it should be [quick-tabS…]. What’s more it looks as though this issue’s been around for at least a couple of years. Seems like it would be an easy enough thing to fix.
Anyway…problem solved 🙂
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By the way, I’ve been finding that at least 9 times out of 10 the Security Code widget on this forum submission page isn’t working. I tried the page in every browser I have here (IE, Safari, Firefox, Chrome) – most of the time, I get either what looks like a broken image icon or the WORD ‘captcha’ where the captcha code should be. FYI.