It must be some kind of interaction with ExtJs because the ExtJs tabs are inside an update panel. Since it works with the previous version I can only assume its some new bug in the new version of System.Web.Extensions.dll
You might inquire about this in the forums on the asp.net site, I've seen posts in the past about issues using ExtJs with MS Ajax.
I've added a setting so you can disable the ExtJsTabs which may help in narrowing it down. If it doesn't disappear with the tabs disabled then its definitely some interaction with ExtJs
<add key="DisableExtJsTabs" value="true" />
The new setting will be in svn trunk by tonight, just add the above to your Web.config or user.config after you do svn update.