/update?secret= had been done - no success.
Deleting the content of bootstrap/cache before copying the files done - no success.
I then moved the whole directory content into another directory and only kept .env and .user.ini, copied the files of 5.9.9 and was able to execute /update?secret. I was on 5.9.9 (flutter) then and I was also able to update from web interface to 5.10.0 even if the updater showed a 500 error message. Maybe because it is react-only now and I started the update from flutter.
So flutter isn’t an option anymore besides by using mobile/desktop app?