This error is caused by the hardcoded connection limit in IIS.
Usually, when you open a web page, the browser opens several connections to the server. On more complex pages (with more controls, images, etc.) the browser needs more time with multiple connections in order to load the page quickly and completely.
Sometimes, a connection might be left half-open, i.e. the browser is closed, but the server still thinks that it is connected to the client. If, for some reason, you reach the IIS connection limit, then the server denies the access to the requested page. In this case you can try resetting IIS (Start >Run...> type "iisreset").
A solution for you could be to increase the number of connection limit of your IIS with 3rd party utility MetaEdit. You can find information about it, and also download it from Microsoft : IIS MetaEdit 2.2 Utility
I hope you find this answers helpful.
the Telerik team