This is happening for all users and only on production environment. 

"Your session has ended. Enter your password to log back in."

Entering your password does not allow you to actually see the admin panel again.

Things we have tried:

<li>Deleting the contents of `craft/storage/runtime`
<li>Removing all rows from `craft_sessions` in db
<li>Setting `'overridePhpSessionLocation' => false`
<li>Setting `'requireMatchingUserAgentForSession' => false`
<li>Setting `craft/app/`, `craft/storage` and `craft/config` as `CHMOD 774`
<li>Changing `craft/` folders to be owned by `www-data`
<li>Setting `session.auto_start = 0`

Our environment is PHP 7 with Ubuntu 16.04 running Apache 2.4.18

Something to consider is we run with an Akamai CDN cache that is set to ignore our entire `/admin` directory, but my suspicion is that something elsewhere is getting cached. Thoughts about that? Is there another directory we should be ignoring?

Also as a point of info, our `/craft` folder is above the `docroot` so not publicly accessible and therefore unlikely to be cached.

Finally, what's very strange is if I remove the modal containing the error message from the DOM in Chrome Inspect, I am able to still navigate the panel fine, without having to re-authenticate.