Editor deletes part of newsletter after plugin and wordpress major upgrade

After i upgraded my wordpress version from 3.6 to 3.8 , and then the Newsletter plugin from 2.5.x to 3.5.x, everytime i use a template to make a new newsletter it deletes part of it (From the middle to end). When i first preview the template with content everything looks good, but when i click on “(2) Go to edit this Message”, this problem happens, and i have to complete it manually. Any ideas on how to get around this? Thank you in advance.

https://wordpress.org/plugins/newsletter/