you should properly escape the form values in the customize tab in the backend. Currently you use template syntax like:
value="<%= ... %>"
This does not escape the value which can break the form fields.
The proper way would be:
value="<%- ... %>"
For example this leads to problems in the send.from.email field. I want to specify the email and name of the sender therefore I enter the following in the form field:
"Firstname Lastname" <email@example.com>
After saving the form and reloading the page the field is broken (due to the use of double quotes)..