We have recently upgraded the LFNS marshals forum and had the same problem there. Apparently getting this to work depends on which version of phpbb is running - some have bugs affecting this. In the LFNS case Roger upgraded the forum to the latest version after moving server. Given that view active topics works on this board in at least one forum it would appear that whatever version is running here is probably good enough.
To fix, as administrator, go into ACP, forums tab, click on the cog wheel for each forum/topic /whatever (not just top level ones) and change "Enable Active Topics" to yes. You have to do this separately for *every* forum and sub-forum.