Method2 - Trying to login using a user whose user group has no assigned role
-Create a user group - Group1
-Under Group1, create a user - user1
(don't assign any role to Group1)
-Try to login in admin interface using the user1
-We are unable to do so, because of a "401 - unauthorized" - ok once again
-Right after, try to login as admin

Here we have a 500 - Fatal Error

{"ErrorMessage":{"_media-type":"application\/vnd.ez.api.ErrorMessage+json","errorCode":500,"errorMessage"
:"Internal Server Error","errorDescription":"User 'user1' doesn't have user\/login permission to SiteAccess
 'site'"}}

*More details*

This is a server firewall error. I you try to refresh the page you get the same error.