Hi,
There are two ways to set Maintenance Mode on or off:
- 
via config.php
- go to your nextcloud folder and then open config/config.php
 - search for 
'maintenance' => true, - change 
truetofalse - save your changes
 - reload your web page
 
 - 
via “occ” command on the command line
- find out the user which runs the web server (usually www-data, http or apache)
- if you don’t know how, copy and paste the following command and execute it:
ps -ef | grep -Ei "apache|nginx" | grep -Ev "grep|root" | awk '{print $1}' | sort | uniq 
 - if you don’t know how, copy and paste the following command and execute it:
 - go to your nextcloud folder with cd (example: 
cd /var/www/nextcloud) - as root user run the following command and replace WEBUSER with the username you just discovered:
sudo -u WEBUSER php occ maintenance:mode --off
(for example:sudo -u www-data php occ maintenance:mode --off) 
 - find out the user which runs the web server (usually www-data, http or apache)