set_error_handler() fails

[icy <auth@xxxxxxxxxxxxx>]

I use set_error_handler() in my script but it fails and returns NULL 
indicating an error has happened.
But there are no entries in error.log ore something similar.
How can I find out what actually went wrong?

PS: I use error_reporting(E_ALL) and init_set('display_errors', 'On') on 
PHP 5.1.3

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