Rtfm browse by topic permissions

I recently installed RTFM 2.2.1, and I have granted group permissions by
article class. I then created a number of topics under each class. The
problem is that for some reason, users get a blank list under “Browse by
topic”. However, if I click on an article with a topic from the Overview
link, and then through to that topic, I can view the topic hierarchy.
I.e. the “Topics” menu gives me
with a blank list, and clicking through articles and onto their topics
gives me with a
hierarchy listing.

I searched the mailing list archives, and didn’t find much at all. The
RTFM online manual
(http://www.bestpractical.com/rtfm/rtfm-2.0-manual-v1.0.pdf) doesn’t
appear to even mention topics.

Am I missing something simple here?