Hi Stipe,
In order to login to the Rights Management End-User console, the user needs to:
1. Have the LiveCycle Rights Management End User role
2. NOT have the Administration Console User role
Either a user is an end-user or an administrator (we don't allow them to be both).
There is also a bug in the preview release where giving a user the Rights Management End-User Role is not reflected until the server is restarted. This bug has been fixed in the 8.0.1 release, but restarting the server after adding the user to the role should workaround the bug for now.
Hope this helps,
-Bill