I had a problem with my zend application hanging with a white screen for no reason.
I narrowed it down to when

$adminSession = new Zend_Session_Namespace('Admin');

was executed, this is just a session? so debugging further, confirmed that session_start() hangs when it used to work.

There are lots of explanations for this, but before you start modifying ini settings, just try this

session_write_close();
session_start();
die(print_r($_SESSION,true));

I found this resolved the problem completely.
I then just removed this debug-code and went back to developing my zend app.