Help migrating

Hi,

I have migrated the site to another URL, where I script installed wordpress so it was all setup. I then FTP copied all the files to the new server, changed wp-config.php to the new databese name, username and password for the DB, imported the mysql database and used search and replace script: https://interconnectit.com/products/search-and-replace-for-wordpress-databases/

I can reach the frontpage (works fine), but displays error warning:
Warning: Cannot modify header information – headers already sent by (output started at /home/snp230/public_html/mju/mju/wp-content/plugins/ht-export-staff/all-staff.php:1) in /home/snp230/public_html/mju/mju/wp-content/themes/govintranetpress/header.php on line 13

No link in the menu is working. If I click a menu it displays:
Not Found
The requested URL /index.php was not found on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
webpageI still get errors

I cannot reach wp-admin. I get this error messege:

Warning: Cannot modify header information – headers already sent by (output started at /home/snp230/public_html/mju/mju/wp-content/plugins/ht-export-staff/all-staff.php:1) in /home/snp230/public_html/mju/mju/wp-includes/pluggable.php on line 1179

Before that I tried changing values in database manually (not via search and replace) and the result was the same.

Can you give me some guidance on what to do. Since the DNS for the previous site is not working anymore I only have FTP access and PHPadmin, no WP-admin access.