"The document server returned an error" when uploading speech-opencgn.0.12.1.folia.xml

When uploading [speech-opencgn.0.12.1.folia.xml](https://github.com/proycon/folia/blob/master/examples/speech-opencgn.0.12.1.folia.xml) to FLAT, it shows error as below:

```
Fatal Error: The document server returned an error
HTTP Error 500: Internal Server Error
The server encountered an unexpected condition which prevented it from fulfilling the request.
```
The latest docker-way running.