Error after Site Transfer

Hi, I just moved a website from one domain to another… the site itself is functioning OK (except for a few custom URLs I need to update). However, I cannot login due to an error.

I get this from /wp-admin/

Warning: Cannot modify header information – headers already sent by (output started at /home/content/k/u/b/kubart/html/wp-content/plugins/category-posts/cat-posts.php:451) in /home/content/k/u/b/kubart/html/wp-includes/pluggable.php on line 890

Any help would be great.