[Plugin: Yoast Breadcrumbs] require_once fails

After updating Yoast Breadcrumbs plugin, the require_once statement fails during activation. So the troubleshooting begins:

At the start of
“readcrumbs/yoast-breadcrumbs.php”

I added this
echo getcwd();echo” x “;echo get_include_path();echo ” x “;echo WP_PLUGIN_DIR;

And got this output:
/home/a/x/ftp_bxcondk/blog/wp-admin x .:/usr/local/lib/php x /home/a/x/ftp_bxcondk/blog/wp-content/plugins
Warning: main(yst_plugin_tools.php): failed to open stream: No such file or directory in /home/a/x/ftp_bxcondk/blog/wp-content/plugins/breadcrumbs/yoast-breadcrumbs.php on line 37

So we are in: “/home/a/x/ftp_bxcondk/blog/wp-admin”
with this include path: “.:/usr/local/lib/php”

No wonder this statement fails:
require_once(‘yst_plugin_tools.php’);

Changing it like this makes it work just fine:
require_once(WP_PLUGIN_DIR.’/breadcrumbs/’.’yst_plugin_tools.php’);

Is this the correct solution? Should this be included in the plugin? Or is there another place to set include path, to help this and other plugins?