'Access Denied' for new users

With the exception of user/1, any new users - regardless of role - cannot access content on my new installation of Open Atrium 2.66; getting redirected to an 'Access Denied' page instead. I have been evaluating Open Atrium, first with a sandbox at Acquia and most recently on my own server (myServer); I set up the same user and group scenario's on both. The Acquia sandbox users are able to access content as expected, however the myServer scenario users can only login and access the recent spaces menu; myServer is not a local machine.

Notably, only spaces the user is assigned to show on the recent spaces menu, but clicking any menu item there results in 'Access Denied'. Also, no main menu items are visible, even if the user is assigned to the space. This is the case on Firefox and Chrome browsers. Edge and Internet Explorer don't even allow a successful login. Although the Acquia sandbox can be accessed from all four browsers.

I used the most recent Open Atrium 2 download from drupal.org for the install.

I've tried:
- Clearing the cache
- Reverting to PHP 5.4 and 5.5 from 5.6
- Clearing the cookies on any browser I used
- adding the myServer url to $cookie_domain in settings.php
- running update.php
- regenerating permissions

Does anyone have any more suggestions?