Contact form send no emails on gantry helium

Support
1

hi there.
i’m new.

The email is not being sent when the submit button is pressed, and the message set in the ‘message’ field is not displayed, nor does it show the page set in ‘display’ field.
Occasionally, after clicking the submit button, the email is sent when the browser is hard-refreshed.
Since the log is output only when the email is sent after a hard reset, there is no error log. i have no idea.

i checked these articles:

the theme is gantry5 helium.
this is the “email.yaml”. i tried both ssl(465) and tls(587)

enabled: true
from: xxx@xxx.com
to: xxx@xxx.com
mailer:
  engine: smtp
  smtp:
    server: mail64.conoha.ne.jp
    port: 587
    encryption: TSL
    user: xxx@xxx.com
    password: xxxxxxxxx
  sendmail:
    bin: '/usr/sbin/sendmail -bs'
content_type: text/html
debug: false
cc: null
bcc: null
reply_to: null
body: null

this is the form data.

title: contact form
form:
    name: contact
    fields:
        name:
            label: Name
            placeholder: Enter your name
            autocomplete: 'on'
            type: text
            validate:
                required: true
        email:
            label: Email
            placeholder: Enter your email address
            type: email
            validate:
                required: true
        message:
            label: Message
            placeholder: Enter your messages
            type: textarea
            validate:
                required: true
    buttons:
        submit:
            type: submit
            value: Submit
        reset:
            type: reset
            value: Reset
    process:
        save:
            fileprefix: contact-
            dateformat: Ymd-His-u
            extension: txt
            body: '{% include ''forms/data.txt.twig'' %}'
        email:
            subject: '[Site Contact Form] {{ form.value.name|e }}'
            body: '{% include ''forms/data.html.twig'' %}'
        message: Thank you for getting in touch!
        display: /contact/thank-you

this is custom HTML on theme outline.

<style>
textarea {
    margin-bottom: 1rem;
}
.button {
    padding: 1rem 3.125rem;
    font-size: 1rem;
}
</style>
<div style="margin: 0 auto; max-width: 750px;">{% include 'forms/form.html.twig' with { form: forms('contact') } %}</div>

Thanks in advance.