UltimateTabstrip does not hard-code document.Form1 dynamically determines the client-side ID of the form containing the UltimateTabstrip control renders it as a parameter to the InitiateUltimateTabstrip method on the client-side. It sounds like your portal might be changing the ID of your form, after the UltimateTabstrip control is initialized on the server-side. When exactly and how does your form change the id from form1 to form1_123?
The value is changed as the form is rendered. It is probably done by this module, which is required to run inside the portal: <httpModules> <!-- AquaLogic .NET Web Controls Support --> <add type="Com.Plumtree.Remote.Loader.TransformerProxy, AquaLogic.WCLoader, Version=220.127.116.11, Culture=neutral, PublicKeyToken=d0e882dd51ca12c5" name="PTWCFilterHttpModule"/></httpModules>
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