Hi Alan, and Welcome to Tikiwiki community!
Unfortunately, I had the same problem on tiki 1.10 for long, and as far as I know, it hasn't been fixed yet.
In my case, I deleted old forums I had created on the same site years ago, and problem solved. The easiest solution (in the meantime there is no coder which can solve it) would be to re-post that message with a slightly different title and/or content. This would (should) do.
And also post a bug report at http://dev.tikiwiki.org/tracker5 so that coders know that this bug is still around (in case you don't find my old bug report in there)