Ho trovato questo commento sul manuale ufficiale di php.net
http://php.net/manual/en/reserved.variables.session.php

When accidently assigning a unset variable to $_SESSION, like

$_SESSION['foo'] = $bar

while $bar was not defined, I got the following error message:

"Warning: Unknown(): Your script possibly relies on a session side-effect which existed until PHP 4.2.3. Please be advised that the session extension does not consider global variables as a source of data, unless register_globals is enabled. "

The errormessage was quite unrelated and got me off-track. The real error was, $bar was not defined.