I've seen similar errors to mine around the forum and Internet, but no solution to my particular problem: When I upgrade plugins using the admin interface, I get this error (some details changed for privacy):
Warning: Cannot modify header information - headers already sent by (output started at /nfs/c05/h03/mnt/707XX/domains/mydomain.com/html/wp-admin/admin-header.php:18) in /nfs/c05/h03/mnt/707XX/domains/mydomain.com/html/wp-includes/pluggable.php on line 868
Most people have said that there is an end php tag missing in some file, but the two files referenced are core files that I haven't modified. This happens after deactivating the old plugin and before removing the old plugin. Any ideas?