No sign-in methods are enabled

This configuration solved the problem:

  RewriteEngine On

  RewriteCond %{REQUEST_URI}      /mattermost/api/v[0-9]+/(users/)?websocket [NC,OR]
  RewriteCond %{HTTP:UPGRADE}     ^WebSocket$ [NC,OR]
  RewriteCond %{HTTP:CONNECTION}  ^Upgrade$ [NC]
  RewriteRule "^(/mattermost/.*)" ws://localhost:8065%{REQUEST_URI} [P,QSA,L]

  <Location /mattermost>
    Require                      all granted
    ProxyPass                    http://localhost:8065/mattermost
    ProxyPassReverse             http://localhost:8065/mattermost
    ProxyPassReverseCookieDomain localhost fqdn
  </Location>
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