Hi Monolo,
Sorry for the inconvenience caused.
If you using MVC 5 project then we suggest you to use the 4.5.1 version of Syncfusion.Ej and mvc5 dll from the following Essential studio installed location.
Syncfusion.Ej: C:\Program Files (x86)\Syncfusion\Essential Studio\14.4.0.20\Assemblies\4.5.1\Syncfusion.EJ.dll
Syncfusion.Ej.MVC: C:\Program Files (x86)\Syncfusion\Essential Studio\14.4.0.20\Assemblies\MVC\Syncfusion.EJ.MVC
For more information about the Assembly combination please refer the following KB.
Regards,
Mahalakshmi K