The problem seems to be that the server doesn't have PEAR installed, which is required for the file manager to work properly.
I replaced the file manager with a different file manager. It seems to work but occasionally it has an error:
You don't have permission to access /webassist/kcfinder/browse.php on this server.
Talk to your hosting provider. See if they can either install PEAR on the server, or if they can fix the intermittent 403 error.