Finally I found the solution in codex….
Interpreting the Error Message:
If the error message states: Warning: Cannot modify header information
– headers already sent by (output started at /path/blog/wp-config.php:34) in /path/blog/wp-login.php on line 42,
then the problem is at line #34 of wp-config.php, not line #42 of
wp-login.php. In this scenario, line #42 of wp-login.php is the
victim. It is being affected by the excess whitespace at line #34 of
wp-config.php.If the error message states: Warning: Cannot modify header information
– headers already sent by (output started at /path/wp-admin/admin-header.php:8) in /path/wp-admin/post.php on line
569, then the problem is at line #8 of admin-header.php, not line #569
of post.php. In this scenario, line #569 of post.php is the victim. It
is being affected by the excess whitespace at line #8 of
admin-header.php.Other issues that might cause that error:
In case you’ve used the function: wp_redirect() or tried to use a
header redirect after the header (or any content at all was sent) that
error message will pop. Instead use javascript redirection if needed.
I removed some extra spaces and everything is working back
add_meta_box(
'pageid_destino',
__( 'ID de Página Destino', MY_THEME ),
'portfolio_custombox',
$posttype,
'normal',
'high'
); //spaces were here