This bug is related to System.Web.Extensions.dll.

Sharepoint uses a Virtual Provider to retrieve content from the database or the file system depending on the type of page (customized or common) and forwards the retrieved page to the asp.net runtime.  When using a ScriptManager (with page methods enabled) on a page that runs in Sharepoint, the ScriptManager maps virtual directories to the file system - and tries to establish a CacheDependency (which uses a FileChangesMonitor to monitor) that specific path/directory. This is what causes the HttpException reported below:

Exception error message when you use a virtual path provider to serve a Web page that has a ScriptManager control: "Directory '<file directory>' does not exist. Failed to start monitoring file changes."

You may refer to the Microsoft Knowledge Base article KB957661 for more details.

There's a hotfix available to resolve this issue. You can obtain it here.