I solved this problem – add in .htaccess 301 rediret manually from non-slash to slash URLs:
<IfModule mod_rewrite.c>
RewriteEngine on
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^(.*[^/])$ /$1/ [L,R=301]
</IfModule>
But I don’t think this is the perfect solution for this problem. Web developers need to test this case, because it occurs on absolutely all sites with WP-FFPC + WP-OPTIMIZE
