Update docs: troubleshooting CORS error

docs/user-guides/installation.md

@@ -297,3 +297,31 @@ After you've saved the changes, run the following command in your terminal to ap
     ```
 
 After this, any new data will be saved directly into the `./data/open-archiver` folder in your project directory.
+
+## Troubleshooting
+
+### 403 Cross-Site POST Forbidden Error
+
+If you are running the application behind a reverse proxy or have mapped the application to a different port (e.g., `3005:3000`), you may encounter a `403 Cross-site POST from submissions are forbidden` error when uploading files.
+
+To resolve this, you must set the `ORIGIN` environment variable to the URL of your application. This ensures that the backend can verify the origin of requests and prevent cross-site request forgery (CSRF) attacks.
+
+Add the following line to your `.env` file, replacing `<your_host>` and `<your_port>` with your specific values:
+
+```bash
+ORIGIN=http://<your_host>:<your_port>
+```
+
+For example, if your application is accessible at `http://localhost:3005`, you would set the variable as follows:
+
+```bash
+ORIGIN=http://localhost:3005
+```
+
+After adding the `ORIGIN` variable, restart your Docker containers for the changes to take effect:
+
+```bash
+docker-compose up -d --force-recreate
+```
+
+This will ensure that your file uploads are correctly authorized.