TESTING

[formentera]

...

[snakes1100]

As its only a "sometimes", its not going to be easy to track down, typically this is cause by a redirect issue in htaccess by url rewrites.

You're going to have to inspect your http logs when it happens or know a time when it happens to see what may be in the logs, a lot of times, you will need debug on in http to even see a error of this type.

When it does happen, rename your .htaccess file to htaccess & see if the error goes away as a quick test.

[webmastersun]

Check your htaccess file or SSL certs which can cause that error.

[Alan_SP]

I recently had just that, on one site I administer.

Site is behind CloudFlare, which causes that issue. I used this to be able to force https:

RewriteEngine On

# If we receive a forwarded http request from a proxy...
RewriteCond %{HTTP:X-Forwarded-Proto} =http [OR]

# ...or just a plain old http request directly from the client
RewriteCond %{HTTP:X-Forwarded-Proto} =""
RewriteCond %{HTTPS} !=on

# Redirect to https version
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]

Found solution here: https://stackoverflow.com/questions/...to-force-https

Not sure will it help you, as I always had "too many redirects" when I tried to force https. This is only thing that worked.

[final kaoss]

This is a very easy fix. I use cloudflare myself and use this to convert http traffic to ssl.. it's ready to go as is.

RewriteEngine on
  #Options +FollowSymLinks
  ################https##############
RewriteCond %{HTTPS} off
RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]
  ################https##############

  <ifModule mod_headers.c>
Header always set Content-Security-Policy "upgrade-insecure-requests;"
</IfModule>