/feed/ problems 500 error

i have a nginx and php-fpm install running wordpress 3.3

i have some problems with the feeds on the sub blogs.

https://validator.w3.org/feed/check.cgi?url=http%3A%2F%2Fitstraining.bloggbyen.com%2Ffeed%2F

feed with problem. all subdomain blogs have this problem.
https://itstraining.bloggbyen.com/feed/

This page contains the following errors:

error on line 29 at column 25: Extra content at the end of the document
Below is a rendering of the page up to the first error.