RESOLVED : I needed to add those assembly references in the web.config too..
<assemblies>
<add assembly="Syncfusion.Core, Version=11.104.0.21, Culture=neutral, PublicKeyToken=632609B4D040F6B4" />
<add assembly="AjaxMin, Version=4.57.4561.28951, Culture=neutral, PublicKeyToken=21ef50ce11b5d80f" />
<add assembly="Syncfusion.Shared.Mvc, Version=11.144.0.21, Culture=neutral, PublicKeyToken=3d67ed1f87d44c89" />
<add assembly="Syncfusion.Linq.Base, Version=11.104.0.21, Culture=neutral, PublicKeyToken=3d67ed1f87d44c89" />
<add assembly="Syncfusion.Theme.Base, Version=11.104.0.21, Culture=neutral, PublicKeyToken=3d67ed1f87d44c89" />
<add assembly="Syncfusion.Tools.Mvc, Version=11.144.0.21, Culture=neutral, PublicKeyToken=3d67ed1f87d44c89" />
</assemblies>