I just posted a comment to the Gemini issue here http://support.dotnetnuke.com/issue/ViewIssue.aspx?ID=14959&PROJID=2

The text of the comment is as follows -

Regardless of SSL (not enabled in the following case), I have a similar issue in a fresh install of 5.6.0. An upgrade to 5.6.1 didn't fix anything. Both the install and the upgrade were clean and error free. Compression and caching are off.

It's a single portal on the installation, with only 7 users. Only the "host" superuser can successfully login. Login attempts with other users (mix of users and superusers) throws the error "Object reference not set to an instance of an object at DotNetNuke.Modules.Admin.Users.Profile.DataBind() at DotNetNuke.Modules.Admin.Authentication.Login.ShowPanel() at DotNetNuke.Modules.Admin.Authentication.Login.ValidateUser(UserInfo objUser, Boolean ignoreExpiring) at DotNetNuke.Modules.Admin.Authentication.Login.UserAuthenticated..." to the log.

Here are some clues to hopefully diagnose -

* the host user doesn't appear in the Host/SuperuserAccounts, yet all of the other users do. They are a mix of users and superusers.

* adding a user via Host/SuperUserAccounts will display the user in the datagrid there, as well as in the Admin/UserAccounts datagrid (logged in as host when creating the superuser).

* conversely, adding a user via Admin/UserAccounts will display the user in the datagrid there, as well as in the Host/SuperUserAccounts datagrid (logged in as host when creating the user).

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Eric Swanzey
www.swanzey.com

Eric;
Have not got a fix for the issue I've lumped all 3 issues that have been reported into 1 Master Bug so we can track this issue more effectively.
http://support.dotnetnuke.com/issue/ViewIssue.aspx?id=15205&PROJID=2

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Ken Grierson
DotNetNuke Corporation
Test Lead