You know maybe this is a bug.
I seem to recall Rick posting this would be fixed after 7.4.2 somewhere but I don't see anything in the change log.
Something to do with some installs not all.
Here is what I found testing this.
Using 7.4.2 One live board one test board.
Both on the same server. Each board is in a separate database but still on the same server.
I cleared cache and deleted the cache files on both.
As far as I can tell all the file permissions are identical and the same goes for CP settings
On the live board the island was rebuilt in cache and it works.
On the test board dead in the water.
In fact top_posters_30.php is not even written in the cache directory.
Here is the post