Error on wp-admin page

Hi,

I am having problems with a website I recently sold. It was working fine on my server and then it was moved to the customers server. I did the install myself, but there is an error appearing. The admin panel cannot be accessed. There is a blank page with a message which sates:

“Warning: Cannot modify header information – headers already sent by (output started at /home/howtoip/public_html/wp-content/plugins/post-layout/plugin.php:611) in /home/howtoip/public_html/wp-includes/pluggable.php on line 868”

The url of the site is the following: https://howtorepairiphone.com/wp-admin/

I can’t see what the problem is. Any help would be greatly appreciated.

Many thanks