This ownCloud instance is currently in maintenance mode, which may take a while.
This is a common message after the upgrade of owncloud instance. The owncloud instance enters the maintenance mode automatically during upgrade. But we need to turn off the the maintenance mode manually after completing the upgrade process.
Step1: Login to the owncloud server via ssh as root.
Step2: Change the location to the folder where owncloud hosted.
Note that the folder path may change according to the environment.
Step3: Turn off the maintenance mode.
#sudo -u www-data php occ maintenance:mode --off
Where 'www-data' is the user which owns the apache process and this may change according to the environment.