returns an array of objects as more than one category can be associated with a post, I guess this could be causing the issue as in your $args2 it is expecting $mycat to be an ID.
You can see what is returned by adding this to your template:
echo '<pre>'; print_r($mycat); echo '</pre>';
If you knew it was always going to be only one term associated with a post you could acces the term_id using:
But it would probably be better to loop through be better to loop through $mycat