Error in plugins.php file with newest upgrade

Am getting the following error (repeated 4x) at the top of the screen when I log in as an admin:

Warning: call_user_func_array() [function.call-user-func-array]: First argument is expected to be a valid callback, 'scd_check_for_plugin_update' was given in /path/to/public_html/wp-includes/plugin.php on line 170

The front end is unaffected and displayer correctly, however when I try and update any of my pages, I get the following immediately after clicking “update”:

Warning: Cannot modify header information - headers already sent by (output started at /path/to/public_html/wp-includes/plugin.php:170) in /path/to/public_html/wp-includes/pluggable.php on line 866

This is followed by a blank screen – no WP toolbar, no menus, nothing. I can go “back” and access the entire backend again, but still cannot add or edit anything.

Please help!
Paul