As a temporary solution:
you can edit featured-content-slider.php file in wp-featured-content-slider direcrory and comment out line 57:
/* add_action(“admin_init”, “c_slider_init”); */
and for remove advert you can remove last line in content-slider.php file in wp-featured-content-slider direcrory:
<p class=”<?php echo $c_slider_class;?>”>Slider by IWEBIX</p>
i have this problem too after updated to v2.6:
Warning: call_user_func_array() [function.call-user-func-array]: First argument is expected to be a valid callback, 'c_slider_init' was given in /home/xxxx/public_html/xxxxx/wp-includes/plugin.php on line 403
and a slider advert link on the slider
