it works fine,
sorry @girish I should have read the Etherpad Cloudron app doc and also the Ownpad one, because I have to enable the detection of pad mimetype by NextCloud https://github.com/otetard/ownpad/#mimetype-detection.
In the terminal, I copied /app/code/resources/config/mimetypemapping.dist.json to /app/data/config/mimetypemapping.json
and then I added in this file pad'': ["application/x-ownpad"],
we could add this information to the Cloudron documentation of the NextCloud app, next time I'll read it 😉