intermittent SSL connection

MAAS has no issues working with SSL. You didn't state the web server you are using, but here is the nginx.conf I used to confirm it works as expected. You most likely are not handing the websocket proxying correctly.

server {
 listen 443 ssl;

 server_name _;
 ssl_certificate /etc/nginx/ssl/nginx.crt;
 ssl_certificate_key /etc/nginx/ssl/nginx.key;

 location / {
  proxy_pass http://localhost:5240;
  include /etc/nginx/proxy_params;
 }

 location /MAAS/ws {
  proxy_pass http://localhost:5240/MAAS/ws;
                proxy_http_version 1.1;
                proxy_set_header Upgrade $http_upgrade;
  proxy_set_header Connection "Upgrade";
 }

}

Seems this is fixed in 2.4, but not 2.3. In 2.3 the following header is returned:

< HTTP/1.1 302 FOUND
< Server: nginx/1.10.3 (Ubuntu)
< Date: Tue, 15 May 2018 19:51:52 GMT
< Content-Type: text/html; charset=utf-8
< Transfer-Encoding: chunked
< Connection: keep-alive
< Vary: Cookie
< X-Frame-Options: SAMEORIGIN
< Location: http://localhost/MAAS/accounts/login/?next=%2FMAAS%2F

It is redirecting to the HTTP instead of HTTPS.

Here is the configuration for apache2:

<VirtualHost *:443>
 SSLEngine On

 SSLCertificateFile /etc/apache2/ssl/apache2.crt
 SSLCertificateKeyFile /etc/apache2/ssl/apache2.key

 RewriteEngine On
        RewriteCond %{REQUEST_URI} ^/MAAS/ws [NC]
        RewriteRule /(.*) ws://localhost:5240/MAAS/ws [P,L]

        ProxyPreserveHost On
        ProxyPass / http://localhost:5240/
        ProxyPassReverse / http://localhost:5240/
</VirtualHost>