Menu block empty

Hello,

i am trying to display three menu blocks cutting my menu in three parts. This is my menu_footer:
-a
--a.1
--a.2
-b
--b.1
-c
--c.1
so i created three menu blocks for that menu, and selecting -a, -b and -c as starting points with Expand all children option, unlimited depth. Then i expected the blocks to show up in my footer... but no way. I tracked down what happens and found out that the blocks are actually ok, but the last call in menu_tree_build function causes everything to disappear. Before line 356:
$data['content'] = theme($hooks, $data['content'], $config, $config['delta']);
the $data['content'] contains my menus. After that call $data['content'] is NULL.

Is some of my modules causing problems with hooks? how can i track it down?

thanks for helping!