Docs: Add workaround against Devise::OmniAuth::StrategyNotFound errors

doc/administration/auth/ldap/ldap-troubleshooting.md

@@ -580,6 +580,25 @@ end
 You can then [run a UserSync](#sync-all-users) **(PREMIUM SELF)** to sync the latest DN
 for each of these users.
 
+## Expired license causes errors with multiple LDAP servers
+
+Using [multiple LDAP servers](index.md#multiple-ldap-servers) requires a valid license. An expired
+license can cause:
+
+- `502` errors in the web interface.
+- The following error in logs (the actual strategy name depends on the name configured in `/etc/gitlab/gitlab.rb`):
+
+  ```plaintext
+  Could not find a strategy with name `Ldapsecondary'. Please ensure it is required or explicitly set it using the :strategy_class option. (Devise::OmniAuth::StrategyNotFound)
+  ```
+
+To resolve this error, you must apply a new license to the GitLab instance without the web interface:
+
+1. Remove or comment out the GitLab configuration lines for all non-primary LDAP servers.
+1. [Reconfigure GitLab](../../restart_gitlab.md#omnibus-gitlab-reconfigure) so that it temporarily uses only one LDAP server.
+1. Enter the [Rails console and add the license key](../../troubleshooting/gitlab_rails_cheat_sheet.md#add-a-license-through-the-console).
+1. Re-enable the additional LDAP servers in the GitLab configuration and reconfigure GitLab again.
+
 ## Debugging Tools
 
 ### LDAP check

doc/administration/troubleshooting/gitlab_rails_cheat_sheet.md

@@ -965,6 +965,9 @@ license.save
 License.current # check to make sure it applied
 ```
 
+This is needed for example in a known edge-case with
+[expired license and multiple LDAP servers](../auth/ldap/ldap-troubleshooting.md#expired-license-causes-errors-with-multiple-ldap-servers).
+
 ### Remove licenses
 
 To clean up the [License History table](../../user/admin_area/license.md#license-history):