Runtime error while logging in/uploading documents

Question

We have a FD services activated intranet SharePoint 2010 site and our external users have been facing problems logging in to the site from their end. They face a runtime error when they want to login:
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, however, be viewed by browsers running on the local server machine.
Details: To enable the details of this specific error message to be viewable
on remote machines, 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 "Off".
<!-- Web.Config Configuration File -->
<configuration>
    <system.web>
        <customErrors mode="Off"/>
    </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="RemoteOnly" defaultRedirect="mycustompage.htm"/>
    </system.web>
</configuration>
We as internal users don't face this issue. What could be a possible reason?

Answer 1

Just an update:
External users are able to log in but get intermittent runtime error messages while trying to upload documents. All users from the same external network are facing this issue.