I found that forums was selected as a legacy buddypress component. I had de-selected it previously but it must not have saved. De-selecting it this time enabled the topic to work again so the problem is fixed.
I find the same problem when I create a fresh installation on the same server. I also have this problem in another wordpress installation on the same server. There was a recent migration of files across to different servers. Is buddypress dependent on particular operating systems?
There seems to be something wrong with the custom header process with the buddypress theme. I set up images with 1250 × 125 pixel dimensions, but they are then cropped and expanded, so part of the image is missing and the resolution is lowered. Most frustrating. Has anyone found this?