I am having the same problem. Just upgraded to 5 and unable to login due to aforementioned view_state error.
Also the login control itself is throwing an error. When I pull the error from the EventLog table I find the following stack trace:
DotNetNuke.Services.Exceptions.ModuleLoadException: The control collection cannot be modified during DataBind, Init, Load, PreRender or Unload phases. ---> System.Web.HttpException: The control collection cannot be modified during DataBind, Init, Load, PreRender or Unload phases.
at System.Web.UI.ControlCollection.Add(Control child)
at DotNetNuke.UI.Utilities.MSAJAX.Register(Page objPage)
at DotNetNuke.UI.Utilities.MSAJAX.RegisterClientScript(Page objPage, String Path)
at DotNetNuke.UI.Utilities.ClientAPI.RegisterClientScriptBlock(Page objPage, String key)
at DotNetNuke.UI.Utilities.ClientAPI.RegisterClientReference(Page objPage, ClientNamespaceReferences eRef)
at DotNetNuke.UI.Utilities.ClientAPI.RegisterClientReference(Page objPage, ClientNamespaceReferences eRef)
at DotNetNuke.UI.Utilities.DNNClientAPI.EnableMinMax(Control objButton, Control objContent, Int32 intModuleId, Boolean blnDefaultMin, String strMinIconLoc, String strMaxIconLoc, MinMaxPersistanceType ePersistanceType, Int32 intAnimationFrames, String strPersonalizationNamingCtr, String strPersonalizationKey)
at DotNetNuke.UI.UserControls.LabelControl.Page_Load(Object sender, EventArgs e)
I tried this using IE7, Firefox 3.0.1 and Google Chrome.
I have a Dutch language pack installed, other then that I am fairly sure the login control is standard.
For now I have rolled backed the installation.