I still got the problem but found out some more details.
a test php and the cron.php worked.
So, I tried to localize the problem in the index.php file by changing certain configurations and thus forcing some checks to raise the error message.
this worked up to here: if (!file_exists('../config.php'))
but not here anymore: if (ini_get_bool('session.auto_start'))
I assume the problem is in between...or completely somewhere else.
I appreciate any help (and others too, as i can see in the forum).