I'm getting a Critical Error every time anyone logs in to my site after upgrading to 5.2 DNN. Bellow is the log for this error. How can I fix this? Any help would be great.

AssemblyVersion: 5.2.0
PortalID: 4
PortalName: Phantom Soccer
UserID: -1
UserName:
ActiveTabID: 99
ActiveTabName: Home
RawURL: /Home/tabid/99/ctl/Login/Default.aspx?returnurl=%2fHome%2ftabid%2f99%2fDefault.aspx
AbsoluteURL: /Default.aspx
AbsoluteURLReferrer: http://www.phantomsoccer.com/Home/tabid/99/ctl/Login/Default.aspx?returnurl=%2fHome%2ftabid%2f99%2fDefault.aspx
UserAgent: Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.1; Trident/4.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; Media Center PC 6.0; .NET CLR 3.0.30618; .NET CLR 3.5.21022; InfoPath.2; Media Center PC 5.0; OfficeLiveConnector.1.3; OfficeLivePatch.0.0; SLCC1)
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider
ExceptionGUID: c16a2d7b-b1ef-4746-9e61-ea8dcb0d569f
InnerException: Object reference not set to an instance of an object.
FileName:
FileLineNumber: 0
FileColumnNumber: 0
Method: DotNetNuke.Common.Utilities.XmlUtils.GetAttributeValueAsInteger
StackTrace:
Message: DotNetNuke.Services.Exceptions.PageLoadException: Object reference not set to an instance of an object. ---> System.NullReferenceException: Object reference not set to an instance of an object. at DotNetNuke.Common.Utilities.XmlUtils.GetAttributeValueAsInteger(XPathNavigator nav, String AttributeName, Int32 DefaultValue) at DotNetNuke.Common.Utilities.Config.GetPersistentCookieTimeout() at DotNetNuke.Entities.Users.UserController.UserLogin(Int32 portalId, UserInfo user, String PortalName, String IP, Boolean CreatePersistentCookie) at DotNetNuke.Modules.Admin.Authentication.Login.ValidateUser(UserInfo objUser, Boolean ignoreExpiring) at DotNetNuke.Modules.Admin.Authentication.Login.UserAuthenticated(Object sender, UserAuthenticatedEventArgs e) at DotNetNuke.Services.Authentication.AuthenticationLoginBase.OnUserAuthenticated(UserAuthenticatedEventArgs ea) at DotNetNuke.Modules.Admin.Authentication.Login.cmdLogin_Click(Object sender, EventArgs e) at System.Web.UI.WebControls.Button.OnClick(EventArgs e) at System.Web.UI.WebControls.Button.RaisePostBackEvent(String EVENT_ARGUMENT) at System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String EVENT_ARGUMENT) at System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String EVENT_ARGUMENT) at System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) --- End of inner exception stack trace ---

Sebastian thanks for the quick response.

The user was the "superuser" account. It does this with every account login with all the portals I have in this DNN. This began to happen after I upgraded to 5.2 . During the upgrade I did have a problem and the site wouldn't show any content under HTML/Text module. The solution I found online was to remove the "HTML" folder under App_Code/. The site worked fine but now I'm getting these login errors.

try to restart application, e.g. by re-saving web.config file.