custom fields problem wit 2.7.1

Hello
I just updated to wp 2.7.1
and I have a problem using custom fields now.
Until now i was using a custom field named img-thumbnail to add a photo by the excerpt on the home page.
now after uploading the photo, i added the custom field as always:
-select on the name of custom fields ‘img-thumbnail’ and then i wrote the path to the file. in this case 2009/04/nameofthefile.jpg
but it’s not working.
the old posts are working fine.
i tried to change the way to described the path to the file: just nameofthefile.jpg
but either way is not working.
can someone help me please?

thank you