I have got this same error message and have tried Googling and Googling and yet found the solution.
First, you have to check your ReportPath
if is correct.
If you're using C#, this code might help:
//declare new report
ReportDocument rpt = new ReportDocument();
//load your report
rpt.Load(Server.MapPath("crMembers.rpt"));
//set your datasource
rpt.Database.Tables[0].SetDataSource(m.GetReportSource());
//set your report for the viewer
crviewer.ReportSource = rpt;
//refresh the viewer
crviewer.RefreshReport();