How can solve that issue?
Hi,
I’d say by providing the right permissions 
You probably followed the Security and Hardening guide and changed the permissions for all files and folders in /var/www/nextcloud, right?
Please check if your web server user (www-data or http) is the owner of files and folders in /var/www/nextcloud
If not run the following commands, but please check what user your web server runs with before that and choose the appropriate command!!
chown -R www-data:www-data /var/www/nextcloud
or
chown -R http:http /var/www/nextcloud
depending on the webserver and defined process owner.
If the permissions are set too strict then you need to run the following both commands:
find /var/www/nextcloud -type f -print0 | xargs -0 chmod 640
find /var/www/nextcloud -type d -print0 | xargs -0 chmod 750
Hope this helps.
which one?
Did you use the updater from the Web GUI or the updater via Command Line Interface (shell)?
If Web then No, if Shell then yes, it suggests.
what is your suggest, YES or NOT
As I wrote, it actually depends. I’m not 100% sure, what happens via CLI update and why Maintenance mode has to be enabled in that case.
Leave it enabled and execute the updater via CLI if not sure.
Thank you very much to all!!! With chown -R www-data: www-data / var / www / nextcloud was solved !!!
Muchas gracias a todos!!! Con chown -R www-data:www-data /var/www/nextcloud quedó solucionado!!!