WordPress Issue: After Installing, I can't post.

WordPress Issue: After Installing, I can't post.

Working on getting our site moved to WordPress and added the Zoho Campaigns Plugin to my active plugins. I was able to generate the API key and figured that out. After creating a simple test form, WordPress loses the ability to post anything new and gives me error strings. 

Warning : Cannot modify header information - headers already sent by (output started at /home/sightli7/public_html/wp-content/plugins/zoho-campaigns/php/signup-formpage.php:1346) in /home/sightli7/public_html/wp-admin/post.php  on line  235

I've been plugging pages into the site and had no other issues until I activated the plug in. When I deactivate the Zoho Campaigns Plugin, the problem goes away. While I can use an HTML field to add the Zoho forms directly into WordPress.