"Could not load type System.Web.UI.WebControls.WebParts.ContentPlaceHolder' from assembly..."
This problem was very annoying: for every second (you read it right) run of my website I got this exception. Next time it would go up fine.
It was very consistent in a very inconsistent way.
Searching for a solution turned out nothing.
After some trail and error I found out this section inside the web.config.
1: <dependentAssembly>
2: <assemblyIdentity name="System.Web.Extensions"
3: publicKeyToken="31bf3856ad364e35" />
4: <bindingRedirect oldVersion="1.0.0.0-1.1.0.0" newVersion="3.5.0.0" />
5: </dependentAssembly>
6: <dependentAssembly>
7: <assemblyIdentity name="System.Web.Extensions.Design"
8: publicKeyToken="31bf3856ad364e35" />
9: <bindingRedirect oldVersion="1.0.0.0-1.1.0.0" newVersion="3.5.0.0" />
10: </dependentAssembly>
My guess is that there is some incompatibility between version 1 and 3.5 and that why "old" web controls like my ContentPlaceHolder got confused.
Anyway commenting this section removed the annoying exception without any strange behavior, in other way of saying it, it worked for me. :)
Ariel