Unrecognized tag prefix CR

I am getting unrecognized tag prefix CR" warning when I compile the code. When I run the solution I am getting below error:

Could not load file or assembly 'log4net, Version=1.2.10.0, Culture=neutral, PublicKeyToken=692fbea5521e1304' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040) Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.IO.FileLoadException: Could not load file or assembly 'log4net, Version=1.2.10.0, Culture=neutral, PublicKeyToken=692fbea5521e1304' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)

Source Error:

< cr:crystalreportviewer id="myCrystalReportViewer" runat="server" autodatabind="true" hascrystallogo="False" width="650px" height="120px" borderstyle="Solid" tooltip="Report Viewer">

Assembly Load Trace: The following information can be helpful to determine why the assembly log4net, Version=1.2.10.0, Culture=neutral, PublicKeyToken=692fbea5521e1304 could not be loaded.

WRN: Assembly binding logging is turned OFF. Does anyone know what is wrong?