Steps to reproduce

1. Start with a JIRA+Confluence OnDemand instance, where the default group is just "users" instead of "jira-users" or "confluence-users".
2. Go to Confluence's General Configuration, change the "Site Homepage" to a space.
3. Unless you've explictly created a group called "confluence-users" and granted it Use permission for that space, you'll receive an error:

The following error(s) occurred:

The XYZ space must be accessible to the confluence-users group (or anonymous view permissions must be granted) in order to be set as the Site Homepage.

Workaround

Manually create a group called "confluence-users" and grant it Use permission for that space. There doesn't need to be anyone actually in the group, it just needs to be granted Use permission.