Returning an error message inside my event receiver using "SPEventReceiverStatus.CancelWithError;" will redirect to "Server Error in '/' Application."

Question

I am working on a team site inside my SharePoint server 2013 on-premise. Now I have two lists:

1. one list is based on the built-in "Issue Tracking" list template.
2. the other list is based on the built-in "Custom" list template.

Now I have added two event receivers; one event receiver will fire when item inside a custom list is updating, while the other event receiver will fire when an item inside an issue tracking list is updating. Inside the 2 event receivers I am doing some checks and if the check fails I will return an error to the user, where inside the 2 lists I am using this code inside itemUpdating method:

public override void ItemUpdating(SPItemEventProperties properties)
        {
base.ItemUpdating(properties);
//checking code goes here
if (checkfailed)
  {
    Errormessage = "Approver is not a valid";

    properties.Status = SPEventReceiverStatus.CancelWithError;       

    properties.ErrorMessage = Errormessage;
  }

Now inside the custom list if the check failed, the user will get the following error on the Edit form Approver is not a valid which is what I need, while inside the issue tracking list if the check failed the user will be redirected to this page:

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. 

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



<!-- Web.Config Configuration File -->

<configuration>
    <system.web>
        <customErrors mode="RemoteOnly"/>
    </system.web>
</configuration>


Notes: 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="On" defaultRedirect="mycustompage.htm"/>
    </system.web>
</configuration>

So can anyone advice on this?

EDIT now i set the CustomError to be off inside the sharepoint web.config file. and i got this error inside the yellow page:-

Answer 1

I highly suspect you actually have another (real) error in the code in case the ER runs in the context of the issue tracking list (e.g. a field missing, an internal name misspelled, ...)
First things first: get the actual error message hiding behind "An application error occurred on the server". Check the ULS log to get it, or check this post to enable full error messages in the browser.