One minute for *any* moodle cron/task job is probably too short ... too often. Many cron jobs/task require more time to complete than 60 seconds allows. Suggest something to the tune of 5 minutes might be more sensible.
Sorry ... don't understand that this means:
"But when all is done, i see that the files are very small, and when I go to the docs, I see that in Moodle 3+
the course files is not backuped."
There is no place, of which I am aware, that the Moodle UI as an Admin level user actually shows all the backup files. There is, however, a report, that indicates if backups were successful and/or the status of automated backups. Is that what you are referring to? On your site: http://yoursite/report/backups/index.php
Have experienced this with one server. Moodle tracks automated backups by writing info to 3 tables. Since the auto backups were failing and I as attempting different combinations/options, was successful in getting a few courses backing up but a couple wouldn't complete when having the automated backups set to cron/task. As a result the tables got messed up ... dates/etc. If your report shows next execution date/time in such a fashion that it will never complete this year (example, next execution set to June 5th and this is already July, you'll have to wait a year before that one might work. So, as a result, I truncated the tables related: mdl_backup_controllers, mdl_backup_courses, and mdl_backup_logs. Truncate means to leave the tables in the database just remove the rows in the tables - the tables contain no data.
That clears all tracking Moodle was doing for backups. It is safe to do.
Then, I doubled checked settings to assure they were set to what I wanted, set automated backups to a sensible time... like 1 AM (not prime time) etc. then waited a day to check. They finally ran and completed successfully and I've not had trouble with that server again.
'spirit of sharing', Ken