I have a similar problem. I have upgraded and now can't login with any of the user accounts that were previously there. I can register a new user and login with them but if I logout and try login again, i get it but I have an error thrown.

A critical error has occurred.
Object reference not set to an instance of an object.

I did find this error in the logs.

AssemblyVersion: 04.05.01
PortalID: 0
PortalName: Email Reaction - Knowledge Base
UserID: -1
UserName:
ActiveTabID: 36
ActiveTabName: Home
RawURL: /DotNetNuke2/ScriptResource.axd?d=MrMmx5_oj-gn94J-kzJGDsuC_4PAftsCzi0sr8p7R-BMDebOcFBRht8G70lbT-Ot0&t=633049125920000000
AbsoluteURL: /DotNetNuke2/ScriptResource.axd
AbsoluteURLReferrer: http://localhost/DotNetNuke2/Home/tabid/36/Default.aspx
UserAgent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.2; SV1; .NET CLR 1.1.4322; .NET CLR 2.0.50727)
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider
ExceptionGUID: e70abc3b-20e6-40ba-b7fe-783e3bcbc86b
InnerException: Unhandled Error:
FileName:
FileLineNumber: 0
FileColumnNumber: 0
Method: System.Web.HttpNotFoundHandler.ProcessRequest
StackTrace:
Message: System.Exception: Unhandled Error: ---> System.Web.HttpException: Path '/DotNetNuke2/ScriptResource.axd' was not found. at System.Web.HttpNotFoundHandler.ProcessRequest(HttpContext context) at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) --- End of inner exception stack trace ---
Source:
Server Name: DAVIDBVS3

David