Hehe. That’s another one, so Nextcloud 9.0.50 and 9.0.51 ship the broken ownCloud updater. To prevent breaking user data it is deactivated in a lot of cases. (if you would have clicked on the button here like you can in ownCloud your instance would get destroyed!)
I just created a revert PR at Revert "[stable9] Don't show the updater if updater is incompatible" by LukasReschke · Pull Request #228 · nextcloud/server · GitHub for this, for you to test this the easiest way would be to change server/apps/updatenotification/controller/admincontroller.php at 49a916fb46a2281aed818d20a9487b6af7e2b1dc · nextcloud/server · GitHub into the following:
‘updaterRequirementsFulfilled’ =>true,
The file for this is apps/updatenotification/controller/admincontroller.php.
That’s correct, unfortunately some things like this check we can’t super easily change. Let’s see what we can do about this to make it as easy as possible for users to update 
Likely it might be an app that can be installed via appstore that ships the corrected code. Also would allow ownCloud users to easily migrate with one click.