This ASP.net error stems from versions of the controls downloaded as part of the Studio for ASP.net Wijmo. This specific error was brought about using the C1ReportViewer. When updating the Studio through C1Live, the tool downloads the following into C:\Program Files (x86)\ComponentOne\Studio for ASP.NET Wijmo\bin\v4 The Wijmo controls, for instance the ReportViewer, may have a reference to the C1.C1Report.4.dll. This may be blind to you. If you add a ReportViewer to your WebForm, it will add a reference to C1.Web.Wijmo.Controls.4.dll to the project. However at Build time, the control will ALSO add a reference to C1.Report.4.dll to the bin folder. Below, as you can see, only the Wijmo.Controls and Controls.Design are being referenced, but in the bin folder, there will be a copy of C1.Report.4.dll… The version of the Wijmo.Controls.dll AND the C1.Report.dll are tied to each other BY VERSION. Therefore, for instance, if you are using the .83 version of the WijmoControls, you will NEED to use the version of the C1.C1Reports that comes when you upgrade the entire studio. If you update the entire studio and use a newer version of the C1.C1Report dll with the older .83 version of WijmoControls, you will get the error above. Rule of thumb: If you update the WijmoControls, make sure you are updating everything else in the project with the newest version of Reports, Excel, PDF, etc.