Hi Philippe,
You’ve updated GP premium plugin to the alpha version, but you didn’t update the GP theme.
I would recommend updating the theme and premium plugin at the same time.
But if your site is a live site with traffic, it’s better NOT update to the Development version (there’re usually some bugs, not sure if your issue is related to this) and wait for the official release.
You can roll back your site or download GP Premium 3.2.4 at https://generatepress.com/account/ .
Let me know if the issue still persists after you roll back the GP Premium version.