I am having the same issue that BlueFrog had.  In fact, this issue seems to be mentioned in several other posts.

I had a 3.2.2 installation that I have attempted to upgrade directly to 4.3.4 which seemingly worked but then I could not log in with old accounts.  And yes, I copied over the decryption and validation keys exactly as they were in the original web.config.

I've restored my 3.2.2 install and then tried going to 3.3.4 with the same result.

Here's the error and stack trace-

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.Security.Membership.AspNetMembershipProvider.UserLogin(Int32 portalId, String username, String password, String verificationCode, UserLoginStatus& loginStatus)

  at DotNetNuke.Entities.Users.UserController.ValidateUser(Int32 portalId, String Username, String Password, String VerificationCode, String PortalName, String IP, UserLoginStatus& loginStatus)     at DotNetNuke.Modules.Admin.Security.Signin.cmdLogin_Click(Object sender, EventArgs e)     at System.Web.UI.WebControls.Button.OnClick(EventArgs e)     at System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument)     at System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument)     at System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument)     at System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData)     at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)     --- End of inner exception stack trace ---

I can create new accounts and login with them just fine.

Any help would be much appreciated.