Probably entirely unrelated, but with my set-up (on certain servers) it was a problem with the 'HTMLEditor/editor/plugins/kfm/.htaccess' file that was causing the problem. Renaming the .htacess file solved my problem.
Thanks for the suggestion - I tried it, but unfortunately this time I just get a blank window with "Please wait...loading".
The fact that renaming htaccess had some effect makes me think it must be related, but I can't think how (?)
One thing I have noticed is the URL in the file manager window that doesn't work is...
...whereas on a file manager that does work (on another server) this URL is...