Failed Login produces System Error

What version are you on? Have you checked the event logs in Kentico? Seems like the login process is erroring out which is why it's redirecting you.

We are on 9. I say it's erroring out when the user incorrectly logs in because the error log message says "Not Authenticated"

Message: Exception of type 'System.Web.HttpUnhandledException' was thrown.
Exception type: System.Web.HttpUnhandledException
Stack Trace: 
at System.Web.UI.Page.HandleError(Exception e)
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean         includeStagesAfterAsyncPoint)
at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean     includeStagesAfterAsyncPoint)
at System.Web.UI.Page.ProcessRequest()
at System.Web.UI.Page.ProcessRequest(HttpContext context)
at ASP.cmspages_portaltemplate_aspx.ProcessRequest(HttpContext context) in     c:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET     Files\root\e0225442\59b957da\App_Web_portaltemplate.aspx.67ab7734.uu-ocm66.0.cs:line 0
at     System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

Message: Not Authenticated.

Exception type: System.Exception
Stack Trace: 
at CMSModuleLoader.CustomSecurityHandler.OnAuthentication(Object sender, AuthenticationEventArgs     e) in         D:\CoffmanTech\Alliance\Kentico\CMS\Old_App_Code\ALS_Custom\CMSExtensions\CMSCustomSecurityModuleLoader.cs:line 74
at CMS.Base.AbstractHandler.CallEventHandler[TArgs](EventHandler1 h, TArgs e)
at CMS.Base.AbstractHandler.Raise[TArgs](String partName, List1 list, TArgs e, Boolean important)
at CMS.Base.SimpleHandler`2.RaiseExecute(TArgs e)
at CMS.Base.SimpleHandler`2.RaiseExecute(TArgs e)
at CMS.Base.SimpleHandler`2.StartEvent(TArgs e)
at CMS.Membership.AuthenticationHandler.StartEvent(UserInfo& userInfo, String username, String password, String passcode, String siteName)
at CMS.Membership.AuthenticationHelper.AuthenticateUser(String userName, String password, String siteName, Boolean login, AuthenticationSourceEnum source)
at CMS.MembershipProvider.CMSMembershipProvider.ValidateUser(String username, String password)
at CMSWebParts_Membership_Logon_LogonForm.Login1_Authenticate(Object sender, AuthenticateEventArgs e) in D:\CoffmanTech\Alliance\Kentico\CMS\CMSWebParts\Membership\Logon\LogonForm.ascx.cs:line 635
at System.Web.UI.WebControls.Login.AttemptLogin()
at System.Web.UI.WebControls.Login.OnBubbleEvent(Object source, EventArgs e)
at System.Web.UI.Control.RaiseBubbleEvent(Object source, EventArgs args)
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

Looks like you should start here with your debugging:

CMS\Old_App_Code\ALS_Custom\CMSExtensions\CMSCustomSecurityModuleLoader.cs : line 74

I'm willing to bet your custom security handler has an error or bad logic in it.

I'm not sure what that is. I am just developing on the web version of kentico. We don't have the VS version.

In the upper right corner when you log into Kentico, there is a ?, click it and it will show you the version.

You will need access to the file system on the server to debug the issue you're experiencing because it's specifically coming from a custom file added as a global handler.

I'm confused as to how we can have a custom file if we did not create it. We don't have access to the .cs files

You may not have put it there but someone did. The error is stating specifically where the file is and what line the error is on (would be on the server running the site).

D:\CoffmanTech\Alliance\Kentico\CMS\Old_App_Code\ALS_Custom\CMSExtensions\CMSCustomSecurityModuleLoader.cs