I was getting this as well and was able to fix it by wrapping a FORM tag around the TabStrip and MultiPage controls. The FORM tag must include the "runat=server" tag property.
These '__[controlname]_State__" controls are ViewState-related, so in order for .NET to create these controls and reference them on PostBack, they must be included inside the form tag. My code that works (no errors when switching between tabs) is below.
One note: an ASPX must not have more than one form tag with a "runat=server" specified, for obvious reasons. You can have other form tags nested (say, within the MultiPage controls); but they cannot have "runat=server" specified. They shouldn't really need them anyway, at least in most cases.
Hope this helps.
Erik
<HTML>
<HEAD>
<title>My Company Web Site</title>
<meta content="Microsoft Visual Studio.NET 7.0" name="GENERATOR">
<meta content="Visual Basic 7.0" name="CODE_LANGUAGE">
<meta content="JavaScript" name="vs_defaultClientScript">
<meta content="http://schemas.microsoft.com/intellisense/ie5" name="vs_targetSchema">
<LINK href="/style.css" type=text/css rel=stylesheet >
</HEAD>
<body leftMargin=0 topMargin=0 rightMargin=0>
<form id="f1" method="post" runat="server">
<P><br><ie:tabstrip id=tsGroupAdmin style="FONT-WEIGHT: bold" TabDefaultStyle="border:solid 1px black;background:#dddddd;padding-left:5px;padding-right:5px;" TabHoverStyle="color:red" TabSelectedStyle="border:solid 1px black;border-bottom:none;background:white;padding-left:5px;padding-right:5px;" SepDefaultStyle="border-bottom:solid 1px #000000;" TargetID="mpGroupAdmin" runat="server" EnableViewState="true" AutoPostBack="true">
<ie:Tab Text="My Plan Information"></ie:Tab>
<ie:TabSeparator></ie:TabSeparator>
<ie:Tab Text="Sublocations"></ie:Tab>
<ie:TabSeparator></ie:TabSeparator>
<ie:Tab Text="Forms/Reports"></ie:Tab>
<ie:TabSeparator></ie:TabSeparator>
<ie:Tab Text="Manage Enrollment"></ie:Tab>
<ie:TabSeparator DefaultStyle="width:100%;"></ie:TabSeparator>
</ie:TabStrip><ie:multipage id=mpGroupAdmin style="BORDER-RIGHT: #000000 1px solid; PADDING-RIGHT: 5px; BORDER-TOP: medium none; PADDING-LEFT: 5px; PADDING-BOTTOM: 5px; BORDER-LEFT: #000000 1px solid; PADDING-TOP: 5px; BORDER-BOTTOM: #000000 1px solid" runat="server" Height="100%" Width="100%" EnableViewState="true">
<ie:PageView>
<asp:Xml id=xmlMyPlanInfo runat="server"></asp:Xml></ie:PageView>
<ie:PageView>
<asp:Xml id=xmlSublocations runat="server"></asp:Xml></ie:PageView>
<ie:PageView>
<asp:Xml id=xmlFormsReports runat="server"></asp:Xml></ie:PageView>
<ie:PageView>
<asp:Xml id=xmlManageEnrollment runat="server"></asp:Xml></ie:PageView>
</ie:multipage></P>
<P> </P>
</form>
</body>
</HTML>