Hi Kunal,
Based on the above error I'd suggest you to look into the following line in the web.config file
Under the node, '<configuration><system.web><httpModules>' for SharePoint 2007 or '<configuration><system.webServer><modules>' for SharePoint 2010, check if the following entry exists. If not, please add.
'<add name="ASPxHttpHandlerModule" type="DevExpress.Web.ASPxClasses.ASPxHttpHandlerModule, DevExpress.Web.v9.3, Version=9.3.3.0, Culture=neutral, PublicKeyToken=b88d1754d700e49a" />'
Also, go to the GAC and ensure that there is an entry there for DevExpress and it matches the above version.
Also, can you try this on a different SP env?
Create a new web/site (using the blank site template) and se how it goes.