Hi,
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
http://www.codeplex.com/smartpart
But haven’t installed asp.net ajax 1.0 version !
Follow it with an iis reset !!
Bye ..
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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.
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