Is it possible to provide a live URL to the problematic page so that we can inspect the problem with Fiddler / Firebug? Please, also send via a support ticket the web.config file so that we can inspect it.
You should also check the server error / events logs for any application restarts when the problem occurs.
The problem could be due to some monitoring program such as WebSense
or a Proxy server
which decrease the URL length of the Dialog handler for example to 1700 symbols. To fix the problem contact your network administrator and increase the allowed URL length to 2000 symbols. You can find more information in these articles: Blank Dialogs Problem
the Telerik team