Patrizio81,
Thank you VERY MUCH for posting this. I had spent 1-2 hours debugging, googling, and searching the forums on this until I found your post.
Changing the functions.php file fixed this for me.
For reference, in WordPress v2.5 I changed line 1388 from:
status_header( 500 );
to
status_header( 200 );
The error messages are working fine now.
Rich