Steps to produce the error? Just visiting the page causes the error?
I'm not getting any error on my local machine nor on the demo site.
Did you use the new Web.config file?
Does Web.config file have this at the top:
<section name="sanitizer" requirePermission="false" type="AjaxControlToolkit.Sanitizer.ProviderSanitizerSection, AjaxControlToolkit" />
and this lower down in the file:
<add name="HtmlAgilityPackSanitizerProvider" type="AjaxControlToolkit.Sanitizer.HtmlAgilityPackSanitizerProvider" />
I have both of those on my local machine and get no errors but I notice on our demo site I apparently did not use the new Web.config so those items are not there, and still no such error on the demo site.
I would say if you don't have that in Web.config then try adding it, but if you do have that then try commenting it out or removing it and see if that makes a difference.
If you remove it from the top make sure you keep this part:
Hope that helps,