Before beta.19.4, Strapi was listening to `0.0.0.0` (which means it listened to all network interfaces) no matter what was specified in the config file `server.js`.
As of beta.19.4, Strapi listens only to the host specified in the config (which is most often `localhost` or `127.0.0.1`).
In some cases (with Heroku, Docker...), listening to `localhost` won't work. In that case, you need to edit your config to specify the correct host to listen to: a specific one if you know it or `0.0.0.0` otherwise.
### Example
**After -** `./config/environments/**/server.js`
```json
{
"host": "localhost",
"port": 1337
}
```
**After -** `./config/environments/**/server.js`
```json
{
"host": "0.0.0.0",
"port": 1337
}
```
## Rebuilding your administration panel
Now delete the `.cache` and `build` folders. Then run `yarn develop`.