I had this error with an old webforms app. Turns out there was a line in the markup causing the issue. I removed it and the error disappeared.
<%@ Register assembly="Microsoft.ReportViewer.WebForms, Version=10.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" namespace="Microsoft.Reporting.WebForms" tagprefix="rsweb" %>