Others may have this problem so I thought I would share as I was at first confused.
Users reported that they could create topics in the forums but they could not read topics whether created by others or themselves.
After trying various options [disabling plug ins etc] I noticed that BuddyPress Forums Component was turned on [ie in Dashboard/Settings/BuddyPress components page. There is a warning not to do this – and a link to this page on this site explaining what to do if you really want Group Forums and Site Wide forums and what to do if you have existing BuddyPress forums rather than bbPress powered ones. https://commonsinabox.org/documentation/buddypress-vs-bbpress-forums
I think that page would be better if it also catered for the situation of those who are NOT migrating from an existing installation but mistakenly thought it would be a good idea to turn on the bbPress powered BuddyPress forum component.
Turning it off appears to have fixed my problem.
[CBOX, WP 3.5.1, BP 1.6.4]