Finally, I have successfully changed the php_value post_max_size variable value to 10M:
php_value post_max_size 10M
in /etc/moodle/apache.conf
and after that on the web interface of Moodle:
Administration / Security / Sitepolicies
maximum file upload size: 3145728
that is 3 MB.
I restarted apache2 and finally I can upload my backup file.
Finally, I have restored the backuped course. Thanks!