Resolve "Include the Crowd application permission information on the "Crowd…

doc/administration/auth/crowd.md

@@ -66,3 +66,15 @@ On the sign in page there should now be a Crowd tab in the sign in form.
 
 [reconfigure]: ../restart_gitlab.md#omnibus-gitlab-reconfigure
 [restart]: ../restart_gitlab.md#installations-from-source
+
+## Troubleshooting
+
+If you see an error message like the one below when you sign in after Crowd authentication is configured, you may want to consult the Crowd administrator for the Crowd log file to know the exact cause:
+
+```
+could not authorize you from Crowd because invalid credentials
+```
+
+Please make sure the Crowd users who need to login to GitLab are authorized to [the application](#configure-a-new-crowd-application) in the step of **Authorisation**. This could be verified by try "Authentication test" for Crowd as of 2.11.
+
+![Example Crowd application authorisation configuration](img/crowd_application_authorisation.png)
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