I had a similar problem… maybe exactly the same as yours.
In my case, it helps me to comment 102 line with phrase
if(!startsWith($dir, "/")) $dir = WP_CONTENT_DIR . "/".$dir; in the
The method called
get_cache_key is responsible for return cache key of themes folder. If we do not comment this line, method will calculate wrong hash (will add unexpected path
Tested on Windows where that path is starts with partition letter 🙂
The plugin is fixed to work with latest version of wp