I was getting this error while trying to all a new custom web part to the site.
Searching for the same found out the reason for that
It happens when you install smart part
But haven’t installed asp.net ajax 1.0 version !
Follow it with an iis reset !!
One thought on “File not found error in NewDwp.aspx SharePoint”
You can also just do an app pool recycle and it will reload the assemblies necessary to make it work again.
Thanks for this fix. I’m not a big fan of SmartPart, only because I like to do it myself.