Warning: filemtime()…

Error message:
Warning: filemtime(): stat failed for style.less in wp-content/plugins/nextend/library/cache/cache.php on line 119

If you see this message, that means that some time ago, and now, you have the free version installed. While you had the full version installed, you choose a theme that is not available in the free version, and those theme’s files can’t be loaded.
Just go to your menus, and save on them once, so the Default theme will be applied to them, and the error message will disappear.