Unable to show login page for NextCloud

NextCloud shows Internal Server Error on first page.

Error

In docker log, shows following message

Class 'OCP\\User' not found

and it points to following file

/data/nextcloud/apps/epubreader/lib/Hooks.php

There was an warning message pointing to epubreader app, shows untruested source, but after I reinstalled it, the message disappered.

Access docker

docker exec -it nextcloudpi bash

Move the epubreader app to /tmp directory.

mv /data/nextcloud/apps/epubreader/ /tmp/

Restart NextCloudPi docker container

After fixed, if try to install EPUB/CBZ/PDF ebook reader app again, following message appears.

Error: This app cannot be enabled because it makes the server unstable

If force install it, NextCloud will appear Internal Server Error again.