Hello Scott
There must be something special on your site causing the issue. Normally, if you select a pre-created mass mail templates, the content of the template will be used to populate message on mass mail form, and you won't get error like that
In this case, submit a support ticket sending us super admin account of your site and link to the page which we can send mass mail and see this error. We are going to check to see why the issue happens and get it sorted
Regards,
Tuan
