Reminders don't work

I happily installed last version of NextCloud on an Ubuntu 18.04.
Everything seems to work well. I can set up events in my personal calendar with reminders (both notification and e-mail) BUT I don’t receive any e-mail nor any notification in my browser (there’s no error: just NO notification and NO e-mail).
E-mail server is set (when I send test e-mail I receive it immediately) and I receive notification in browser when I use Talk.
My user is administrator, how can I understand what’s happening?
Thanks for any hint.

Make sure it’s activated in the Groupware Admin Settings and that cron is running often enough.

Thanks, in Groupware Admin Settings I’ve got all 4 boxes checked.
I am using cron service to invoke cron.php and I have:
-rw-r–r-- 1 www-data www-data 5048 Mar 11 15:52 cron.php

I noticed that when I add a reminder to my events and then try this command:
sudo crontab -u www-data -l
I get
no crontab for www-data

So I suspect that reminders really don’t go into cron jobs…

If you don’t have any cron for the www-data then you don’t use system cron. Set up a system cron and change mode in the admin settings.

Thank you!
I have put this line
*/1 * * * * php -f /var/www/html/nextcloud/cron.php
in www-data cron jobs, restarted cron service and … voilà!
It works (thank you, I thought that line was added dinamically)
I ask you a last advice:
Is it too much “every 1 minute”?
Thanks again

Yes, launching system cron every minute is too much. You should launch it every 5 minutes.

However if you want to be sure to have reminders on exact time you can use a dedicated cron command for which you can increase the frequency: (sorry, documentation is not yet merged).

Ok, thank you very much!