have a serious problem that I can't solve and I need help quick.

I was not very smart and didn't backup the sql server database when I decided to upgrade.  I did backup the site.

I did the upgrade by copying the unzipped files over the top of the existing site.  Then I accessed the site and it ran through the upgrade.  When I went to the site all was well until I tried to login.  No matter who I try to login as, I get this error and I have no idea how to solve it.  I do have some modules installed from third party of which one is MPUS multi-portal user.  There are about 4 portals in this site.  Because I can't login, I have no way to get into the site to remove modules, etc...

Here is the error I am getting:

Can anyone help me solve this quickly

Server Error in '/' Application.
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Item has already been added. Key in dictionary: 'UserInfo'  Key being added: 'UserInfo'
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.ArgumentException: Item has already been added. Key in dictionary: 'UserInfo'  Key being added: 'UserInfo'

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:

[ArgumentException: Item has already been added. Key in dictionary: 'UserInfo'  Key being added: 'UserInfo']
   System.Collections.Hashtable.Insert(Object key, Object nvalue, Boolean add) +2836883
   System.Collections.Hashtable.Add(Object key, Object value) +11
   DotNetNuke.HttpModules.Membership.MembershipModule.OnAuthenticateRequest(Object s, EventArgs e) +3089
   System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +92
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +64
 

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Version Information: Microsoft .NET Framework Version:2.0.50727.42; ASP.NET Version:2.0.50727.210