Just include the path to the view, with the file extension.
Razor:
@Html.Partial("~/Views/AnotherFolder/Messages.cshtml", ViewData.Model.Successes)
ASP.NET engine:
<% Html.RenderPartial("~/Views/AnotherFolder/Messages.ascx", ViewData.Model.Successes); %>
If that isn't your issue, could you please include your code that used to work with the RenderUserControl?
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