500 Code in Apache log because of Websocket

I resolved my issue using advice from this thread. In essence, It required the addition of the mod_proxy_wstunnel. In Jessie, I did it with:

sudo a2enmod proxy_wstunnel

For posterity, here's the working config:

<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>