This Nextcloud instance is currently in maintenance mode, which may take a while.
This page will refresh itself when the Nextcloud instance is available again.
Contact your system administrator if this message persists or appeared unexpectedly.
Thank you for your patience.
From you nikname, @System_Admin suppose that users should contact you 
Welcome to the Forum! If you, please, use template for a new support question and give community more information about your Environment and issue then you will, probably, get an answer.
Meanwhile you can read some documentation about Maintenance mode.