Trying my app on another Android phone the next day, I noticed I had no more error. I removed and reinstalled the app on the other Android phones and the plugin also started working fine on all of them.
I'm not sure what the problem might have been: issue with hydra plugin (now I know if something weird happens I may have to remove and reinstall the app, and make sure the hydration is complete), needed more time after creating the app on facebook (even if it's stated it can take minutes, and not hours, to update all the servers), ...