This usually pops up when Odoo can't properly update the module list due to permission issues. Let's break down how to fix it step by step:

### **Quick Fix (Most Common Solution)**

1. **Restart the Odoo Server**

- Sometimes, a simple server restart resolves it.

- If using a terminal:

```bash

sudo service odoo restart # For systemd (Ubuntu)

```

- Or kill and restart manually if running in dev mode.

2. **Check File Permissions**

- Ensure the Odoo user has write access to the `addons` folder.

- Run:

```bash

sudo chown -R odoo:odoo /path/to/odoo/addons

sudo chmod -R 755 /path/to/odoo/addons

```

3. **Force Update via Command Line**

- If the UI fails, try updating the module list manually:

```bash

odoo-bin -d your_db_name --update=all --stop-after-init

```

---

### **If the Error Persists**

- **Check the Odoo Logs** (`/var/log/odoo/odoo-server.log`) for more clues.

- **Database Corruption?** Try creating a new test database to see if the issue is DB-specific.

- **Proxy/Network Issue?** If behind a proxy (like Nginx), ensure requests to `/web/database/selector` aren’t blocked.

---

### **Pro Tip**

If you’re in **development mode**, this error can sometimes be ignored—just refresh the page or clear your browser cache (`Ctrl+F5`).