[Plugin: NextGEN Gallery] Error after upgrade (call_user_func)

Hi.
I have recently upgraded to wp 2.7 and updated NextGen too.
I just noticed that if use Upload/Insert image in a post, I get in the fist screen (“From Computer”) the following error message:

Warning: call_user_func_array(): First argumented is expected to be a valid callback, ‘media_admin_css’ was given in /home/content/c/y/b/username/html/talk/wp-includes/plugin.php on line 339

The rest of the button/options show, but they are a bit “folded” below the message.The upload seems to work too. But why the error message?

Can you please help to resolve this?
Many thanks