If you are using the code Page.Load("control_path..."). make sure that the actual ascx file you load does not have any directives that include the assembly from your exception message
<%@ Register Tagprefix="SharePoint" Namespace="Microsoft.SharePoint.WebControls" Assembly="Microsoft.SharePoint, Version=14.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c" %>