It was my first upgrade and by Change, I’ve done it wrong. Now I have to deal with it, but I don’t know how.
The browser give me the error message “The files of the app (viewer) were not replaced correctly. Make sure it is a version compatible with the server.”
If I perform “./occ upgrade” I’ll get:
“UnexpectedValueException: The files of the app “viewer” were not correctly replaced before running the update”
But I can not see any app “Viewer”. Neither in the app Directory, nor if I try to upgrade with “./occ app:update Viewer”
I tried to find some Information how to deal with this Kind of error, but mostly the descriped apps could be found in the app Directory.
You can try download the app viewer from https://download.nextcloud.com/server/releases/ (pick the NC version you’re already using). From the downloaded archive, open the app folder : you’ll see the viewer folder. Copy this entire folder to your original nextcloud app/ folder, run again occ upgrade, and you should be good to go.
Have the same problem during an upgrade from 18.0.4 to 18.0.12.
The solution from @armymen did not work for me as the releases 18.0.12 or 18.0.4 doen’t have a app called “viewer” included
I simply removed “viewer” from the AlwaysEnabled in ´core/shipped.json´ and did the upgrade without problems. Later version upgrades reinstalled this automatically. May break things, I don’t know!