Access Control issue in CiviCRM with Groups?
I can confirm the warning on a vanilla CiviCRM install. Creating an ACL for the mailing list group(s) fixes it, but that doesn't seem right because:


  The anonymous role (public in Joomla!) applies to all visitors to the
  website who have not logged in. This role will have the lowest level
  of permissions. The default CiviCRM permissions for this role are
  [...] subscribe and unsubscribe from mailing lists
  https://docs.civicrm.org/user/en/latest/initial-set-up/permissions-and-access-control/


So in principle this shouldn't be necessary.

The problem actually lies in the CRM_ACL_BAO_ACL::group() method because the cache returns NULL rather than an empty array. This then gets passed back unchecked to CRM_ACL_API::groupPermission() where the warning is issued.

Two options for you then:


Raise an issue on Jira and get the problem fixed
Suppress warnings and write them to a log file instead


Doing both would be the responsible way to go :-)