Box.net has dynload=false, which means that on each request the whole tree is retrieved and returned to filepicker.
It works ok in JS filepicker because it caches the tree and when user selects a folder no request to server is sent

But in non-JS filepicker the request for each subfolder is sent but boxclient::getfiletree ignores argument $path.

Also I receive a warning:

Warning: htmlspecialchars() expects parameter 1 to be string, array given in /var/www/repositories/master/moodle/lib/weblib.php on line 100

I traced it back and noticed that boxclient::buildtree returns an array in 'path' attribute instead of a string