Jean-Marc,
From my experience polls is only working up to 0.8.1 and only up to ownCloud 10.0.8
On the repository GitHub - nextcloud/polls: 🗳️ Polls app for Nextcloud they write
Note : ownCloud is no longer supported! Last (confirmed) working version is 0.8.1 and is released in the oC marketplace.
So this app is abandoned for ownCloud until somebody takes the effort and creates a working fork.