AJAX link error (form API)
While I still don't know where the problem is (it should work this way) I've found a solution. 

Instead returning the whole $page_array in callback function and hoping that Drupal will understand it and replace the div specified in '#ajax' =&gt; array('wrapper' =&gt; 'TSTDiv',... property of the link with it you can simply use ajax commands and 'return' something like this:

$commands[] = ajax_command_replace('#TSTDiv', render($page_array));
$page = array('#type' =&gt; 'ajax', '#commands' =&gt; $commands);
ajax_deliver($page);


And vuala it works just fine (and looks more professional at the same time!).

Would still appreciate and answer why the original approach didn't work, though.

BTW: I didn't know the AJAX commands before.