Update FAQ.md

Signed-off-by: Jan-Otto Kröpke <[email protected]>

docs/FAQ.md

@@ -12,11 +12,31 @@ we encourage users to up-vote the relevant feature requests on the OpenVPN GitHu
 You can find and support these requests at the following link:
 [Feature Request on GitHub](https://github.com/OpenVPN/openvpn/issues/299)
 
-
 ## username-as-common-name
 
-When configuring `username-as-common-name` on the OpenVPN server,
-it's essential to ensure that `openvpn.common-name.environment-variable-name` is also set to `username`.
+When setting up `username-as-common-name` on the OpenVPN server, it's crucial to also configure `openvpn.common-name.environment-variable-name` to `username`.
+
+This configuration is indispensable because `username-as-common-name` functions post-authentication. Aligning the environment variable name with `username` guarantees smooth operation.
+
+## Options error: No client-side authentication method is specified.
+
+Although openvpn-auth-oauth2 theoretically doesn't require client-side authentication, the OpenVPN client expects it.
+
+**Upstream Issue:** [GitHub Issue #501](https://github.com/OpenVPN/openvpn/issues/501) (Please react with :+1: if you're affected.)
+
+**Potential Workarounds:**
+
+1. **Configure Client Certificates**
+   Implement client certificates to enable client-side authentication.
+
+2. **Utilize Inline auth-user-pass**
+   OpenVPN accepts `auth-user-pass` for client-side authentication. You can define the username and password inline to prevent the OpenVPN GUI from requesting a password.
+
+   ```
+   <auth-user-pass>
+   username
+   password
+   </auth-user-pass>
+   ```
 
-This configuration is mandatory because `username-as-common-name` operates after the authentication process.
-Matching the environment variable name to `username` ensures seamless functionality.
+   Note: The username/password can be any dummy value as they won't be validated by openvpn-auth-oauth2 or OpenVPN itself.