Web.config error when saving an Event Rule

If you are saving/exporting an event rule, and you receive this error:

Server Error in '/' Application.
Runtime Error

Description

An application error occurred on the server. The current custom error settings for this application prevent the details of the application error from being viewed remotely (for security reasons). It could be viewed by browsers running on the local server machine (localhost).

Enabling more details

To enable the details of this specific error message to be viewable on remote machines, create a <customErrors> tag within a web.config configuration file located in the root directory of the current web application. This <customErrors> tag should have its mode attribute set to Off.

Example

<!-- Web.Config Configuration File --> 
<configuration>
<system.web> <customErrors mode="Off"/> </system.web>

</configuration>

The current error page you are seeing can be replaced by a custom error page by modifying the defaultRedirect attribute of the application's <customErrors> configuration tag, to point to a custom error page URL:

<!-- Web.Config Configuration File --> 
<configuration>
<system.web> <customErrors mode="RemoteOnly" defaultRedirect="mycustompage.html" </system.web>
</configuration>

How to fix the problem

Read how to fix the potentially dangerous request error