This description leads to two different errors, but i'll join them as one issue because they might have the same root.

Method1 - Trying to login using an unauthorized "guest" user
-Create a guest user - user1
-try to login in admin interface using the guest user (user1)
-We are unable to do so, because of a "401 - unauthorized" - ok, this is expected
-Right after, try to login as admin

Here we have an infinite spinning wheel and in the browser developer tools the error

TypeError: this.get(...).one(...) is null[Learn More]  _ezcombo:717:37

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'"}}

In both cases, if we clear browser caches, we are again able to login as admin.

As said in the beginning of description, I opted to open only one issue, but if the root cause is different, please do say so that i'll separate this issue in two.