Feed error, Unable to access /xxxx/wp-includes/pomo/entry.php

Hi, since yesterday I get this error in the feed, after few days it was working fine (I made no change):

Warning: require_once() [function.require-once]: Unable to access /xxx/wp-includes/pomo/entry.php in /xxx/wp-includes/pomo/translations.php on line 10

Warning: require_once(/xxx/wp-includes/pomo/entry.php) [function.require-once]: failed to open stream: No such file or directory in /xxx/wp-includes/pomo/translations.php on line 10

What could be?
Thank you