PHP Error

Last days I made several modifications of the original PHP code of WP. Afterwards I realized that there are error entries in my error log with almost any HTTP request to my blog (except RSS files and some other stuff out side the WP HTML pages). The error message is:

PHP Warning: call_user_func_array() [function.call-user-func-array]: First argument is expected to be a valid callback, ‘feed_links_extra’ was given in /home/polkomm/htdocs/blog/wp-includes/plugin.php on line 339

That’s line 339 which is part of the function do_action:

call_user_func_array($the_[‘function’], array_slice($args, 0, (int) $the_[‘accepted_args’]));

I have re-done all of my code modifications (at least I hope so). As my server provider recommended I have also de-activated the only plus-in I am useing, Akismet (which deletes spam postings). But unfortunately the error remains. I am wondering if it is not my PHP code which causes the problems.

Is there anybody who can give me a hint how I can fix that?

Thanks
Tom