Submitting new message remains gray

You are correct that the problem seems to be from websockets.In the error log for apache, I see:

[Wed Jun 15 17:00:39.470705 2016] [proxy:warn] [pid 16801] [client x.x.x.x:57356] AH01144: No protocol handler was valid for the URL /api/v3/users/websocket. If you are using a DSO version of mod_proxy, make sure the proxy submodules are included in the configuration using LoadModule.

I'm not entirely sure what to do with this. Here's my current apache SSL file, using let's encrypt:

<VirtualHost x.x.x.x:443>
ServerName my.mattermost.org
SSLCertificateFile /etc/letsencrypt/live/my.mattermost.org/fullchain.pem
SSLCertificateKeyFile /etc/letsencrypt/live/my.mattermost.org/privkey.pem
Include /etc/letsencrypt/options-ssl-apache.conf

ProxyPreserveHost On
ProxyRequests Off

RewriteEngine On
RewriteCond %{REQUEST_URI} ^/api/v3/users/websocket [NC,OR]
RewriteCond %{HTTP:UPGRADE} ^WebSocket$ [NC,OR]
RewriteCond %{HTTP:CONNECTION} ^Upgrade$ [NC]
RewriteRule .* ws://127.0.0.1:8065%{REQUEST_URI} [P,QSA,L]
RewriteCond %{DOCUMENT_ROOT}/%{REQUEST_FILENAME} !-f
RewriteRule .* http://127.0.0.1:8065%{REQUEST_URI} [P,QSA,L]
RequestHeader set X-Forwarded-Proto "https"

  <Location /api/v3/users/websocket>
    Require all granted
    ProxyPassReverse ws://127.0.0.1:8065/api/v3/users/websocket
    ProxyPassReverseCookieDomain 127.0.0.1 my.mattermost.org
  </Location>

  <Location />
    Require all granted
    ProxyPassReverse https://127.0.0.1:8065/
    ProxyPassReverseCookieDomain 127.0.0.1 my.mattermost.org
  </Location>
</VirtualHost>