This plugin is sniffing and adding vars to posted data..and can cause issues

[mrbm]

I found out the hardway and lost 4 days finding this...yikes.

In this file you will see it sniffing and adding variables:
https://github.com/sendgrid/wordpress/blob/master/lib/class-sendgrid-mc-optin.php

This becomes an issue in home_url scenarios, it can cause the jquery .load function to fail.

This sendgrid service is very valuable, so to keep using the service with this plugin, you can simply comment out all includes that reference this file in the files:

lib/class-sendgrid-settings.php
wpsendgrid.php

This will effect their opt-in widget, but there are other options that might be better and updated for doing opt-in widgets.