I moved from WSP to SolidCP, all was quite ok but when i login to the panel I get this error

Exception type: NullReferenceException
Exception message: Object reference not set to an instance of an object.
at SolidCP.Portal.Login.LoginUser(String username, String password, Boolean rememberLogin, String preferredLocale, String theme)
at SolidCP.Portal.Login.btnLogin_Click(Object sender, EventArgs e)
at System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument)
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

If I remove the /error.htm and I reload the page, the panel is working fine.

What can it be?

Thanks, Giuseppe